When starting out a great place to start is Vanguard. They are the company who invented the index tracker fund, and they have a long history of aiding the little guy in playing the investing game on good terms.

When your investments reach around £100k, a flat fee platform such as Interactive Investor becomes cheaper than Vanguard.

While you are starting out and have very little to invest, Vanguard may not be the cheapest choice, since they have introduced a minimum £4 per month fee. An alternative may be Hargreaves Lansdown, or at the time of writing there is a free platform from InvestEngine – but I believe they are a loss-making startup, which raises the question for me of how long their financial backers will continue to support a loss-making operation, or whether your business will perhaps be sold on to someone else?

When opening an account with a platform, be alert for scams. Check that the platform is genuine, and that you are going to their real website.