The Money Market Fund Boom: Why £18 Billion Flowed Into UK MMFs in 2026 — and the Risk Most Investors Miss

Open the platform statements of any UK self-directed investor under 45 and there's a good chance you'll find a money market fund sitting in the cash position. A year ago, MMFs were where pension funds parked their treasury reserves. In 2026, retail platforms have made them mainstream — and the inflows have been startling. Hargreaves Lansdown reported £8.3 billion of net retail flows into MMFs across the first quarter alone. AJ Bell came in at £4.1 billion. interactive investor and Vanguard UK added another £6 billion between them.

The pitch is simple and almost too good. You earn 4.6%–4.85% AER, the fund holds short-dated government and high-grade corporate paper, you can sell at the daily price, and there's no fixed term locking you in. Compared to a 4.5% easy-access savings account, the MMF wins on yield. Compared to a 4.7% one-year fixed-rate cash ISA, the MMF wins on flexibility. So why isn't everyone using them already?

What an MMF actually is in 2026

An MMF is a regulated open-ended investment company that holds a portfolio of very short-dated, high-quality interest-bearing instruments. The biggest UK retail funds — Royal London Short Term Money Market, Legal & General Cash Trust, Fidelity Cash Fund, BlackRock ICS Sterling Liquid Environmentally Aware Fund — typically hold:

• UK Treasury bills (gilts maturing in under 12 months)
• Bank certificates of deposit (CDs)
• Commercial paper from highly-rated corporates
• Repurchase agreements (repos) backed by gilts
• Term deposits at major banks

The weighted average maturity (WAM) is usually under 60 days. The credit quality is investment grade or better — typically rated A1/P1 by S&P and Moody's. The fund pays out the running yield of those underlying instruments minus a small fee — usually 0.10% to 0.20% for the cleanest retail share classes.

Since the European Money Market Fund Regulation came into force, MMFs are split into three legal forms. UK retail investors generally see two of them: VNAV (Variable Net Asset Value) funds where the unit price floats slightly day to day, and LVNAV (Low Volatility NAV) funds which target a stable £1.00 unit price but can break that target in stressed markets. The third type, public-debt CNAV (Constant NAV) funds, are mostly institutional.

The yield story in 2026

Why is the yield so high right now? Two reasons. First, the Bank of England base rate is still 4.25% in May 2026, after the long climb from 2022. Sterling Overnight Index Average (SONIA) — the benchmark MMFs typically beat by 5–15 basis points — is sitting at 4.20%. Second, short-end gilts and three-month bank CDs are pricing slightly above SONIA because the market has only modestly priced in further base rate cuts.

That gives the cleanest MMFs a gross running yield of 4.85% to 5.00%, which after the 0.10–0.15% fund fee leaves the investor with 4.7% to 4.85%. Compare that to a current account paying 0.5% and you can see why money is moving.

The current best yields on platform

Five funds dominate UK self-directed flows in 2026:

• Royal London Short Term Money Market Fund (Class Y Acc): 4.78% gross running yield, 0.10% OCF
• Legal & General Cash Trust (Class C Acc): 4.72% gross running yield, 0.15% OCF
• Fidelity Cash Fund (Class W Acc): 4.74% gross running yield, 0.15% OCF
• Vanguard Sterling Short-Term Money Market Fund: 4.69%, 0.12% OCF
• BlackRock ICS Sterling Government Liquidity Fund: 4.63%, 0.10% OCF (gilts only — lowest credit risk)

Where the risk actually hides

Here's the thing though. MMFs are not cash. The Financial Conduct Authority does not classify them as cash deposits. There is no FSCS protection. An MMF is technically an investment, and the unit price can fall.

This isn't theoretical. In March 2020, when COVID-19 hit credit markets, a handful of European MMFs experienced "breaking the buck" pressure — the LVNAV £1 unit price wavered as redemption pressure forced funds to sell their longer-dated CD holdings into a no-bid market. The Bank of England and ECB ultimately backstopped the system through emergency facilities, but a small number of institutional MMFs were briefly suspended. Retail investors generally weren't affected — but the regulatory wake-up call led to the 2024 reforms now in force.

Under those reforms, all UK-domiciled MMFs must hold at least 30% of their assets in instruments maturing within seven days, and 7.5% in instruments maturing within one day. They can also invoke a redemption gate or liquidity fee in stressed conditions — though the FCA can override this in a crisis. The risk isn't large, but it isn't zero, and it's qualitatively different from FSCS-protected cash.

The platform layer matters

And here's a wrinkle most retail investors miss entirely. When you buy an MMF on Hargreaves Lansdown or AJ Bell, the platform earns money from the cash interest on uninvested cash you hold in your dealing account. If you switch your cash from the platform's cash interest scheme into an MMF, the platform loses that revenue. Some platforms now charge an MMF holding fee or a higher dealing charge to compensate. Read the platform fee schedule before you switch — it can shave 0.20% off your effective yield. Vanguard UK, Trading 212 and InvestEngine charge nothing extra; HL charges its standard 0.45% platform fee on funds, which on a £20,000 holding is £90 a year. Net of platform fees, HL holders may end up only marginally ahead of an FSCS-protected easy-access account paying 4.5%.

Where the MMF beats both cash and gilts

The use case is specific and real. If you have £30,000+ sitting in an ISA wrapper as a cash position — perhaps because you've sold equities and want to wait for a better entry point — an MMF earns the full short-rate yield with daily access, inside the tax-protected wrapper. A cash ISA is generally limited to £20,000 per tax year and may not pay competitive rates inside an investment platform. A short gilt fund is similar in yield but with more interest rate risk if the Bank of England moves unexpectedly. The MMF threads the needle.

For SIPP holders, the case is even stronger. Most SIPPs pay close to 0% on uninvested cash — sometimes literally nothing. An MMF inside a SIPP turns dead capital into something earning 4.7% gross. Over a £100,000 cash position waiting to be deployed, that's £4,700 a year of pension growth that would otherwise vanish.

Who should pause before clicking buy

If your money is for everyday spending, your emergency fund, or anything you might need in the next two weeks under stress conditions, FSCS-protected easy-access cash is a better fit. The 0.2% yield difference isn't worth the small but real chance of an MMF gating in a future crisis. If your time horizon is longer than 18 months, a one-year fixed-rate cash ISA or a short-dated gilt may pay slightly more.

The MMF sweet spot is between those two: medium-term parking of investment cash inside a tax wrapper, where FSCS protection is irrelevant anyway because the platform itself is the access point. For that use case in 2026, MMFs are the cleanest option on the table — provided you understand what you're buying.