There are a huge range of platforms that you can to invest. All offer something different, cheaper fees, more investment options, better websites, better customer service. You may already have accounts in company pension schemes too that you could use to invest further.
For simple tracker funds, AJ Bell and Vanguard often come out as hihgly recommended and with reasonable fees and options. HL has a great app and website, but is considerably more expensive. Keeping fees low is very important, as is explained in another post.
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