I have a small sum in a Stocks and Shares ISA with Hargreaves Lansdown, which I only started in the last tax year. I believe it's reputed to be among the more expensive platforms, but I looked at the charging structure of the various platforms, got bamboozled (my brain doesn't handle that sort of information very well), and decided that not investing at all could be the most expensive choice of all.
I chose BestInvest for my husband's SIPP, also on the basis of, "narrow it down to a handful of low cost options with user-friendly websites, do eeny-meeny-miney-mo, and get on with it."