Index funds (OEICs): the once-a-day workhorse
You buy units from the fund company itself, priced once a day after markets close. No spreads, no premiums, no live prices to watch, which for a monthly saver is a feature: you cannot fiddle with what only trades daily. For automated monthly investing into a global tracker, the humble OEIC is often the cleanest tool in the box.
ETFs: the same idea, live on the exchange
An ETF holds the same kind of basket but trades on the stock exchange like a share: live prices, buy and sell any minute. Costs are often a shade lower, and on flat-fee-capped platforms large sums can be dramatically cheaper to hold as ETFs.
The plumbing to respect: you pay a bid-offer spread each trade (tiny on giant ETFs, real on niche ones), and intraday tradability is a temptation dressed as a feature. Buy the same day each month and the difference from a fund rounds to nothing.
Investment trusts: the old, odd, interesting one
A trust is a company whose business is owning investments; you buy its shares. Because the share price and the value of what it owns (NAV) are set by different forces, trusts trade at discounts or premiums to their assets, sometimes wide ones. They can also borrow to invest (gearing), which amplifies both directions, and can hold back income in good years to keep paying dividends in bad ones, which is how some have raised dividends for fifty-plus years.
That makes trusts genuinely useful for income strategies and illiquid assets, and genuinely riskier than they look when bought at a premium with gearing. Know the discount and the gearing before buying; both are published.
Choosing without agonising
Monthly saver on a percentage-fee platform: the index fund is perfect. Larger pot on a platform that caps fees for exchange-traded holdings: the ETF equivalent may be materially cheaper. Income focus or specialist assets: trusts earn their oddness. All three at global-tracker level are more alike than different, and the worst choice is deferring investing while comparing them.