Platform costs depend on what your holdings are (Individual Shares, ETF's, OEIC's, Unit Trusts etc), how there are held (Unwrapped, SSISA, Pension), the total value of your funds, and how much you trade/churn/buy/sell per week/month/year. So there is no single answer that applies to everyone. I do remember there was a niche where HL could work out cheapest, but I can't remember what that niche was - all I knew is it didn't work for me. I transferred to Interactive Investor, but I don't hold any individual shares so they may not be cheapest for anyone else.
There is a spreadsheet run by "Snowman" over on the MoneySavingExpert website/forum. It allows you to enter everything you hold, and it shows you what the current charges are for a multitude of Platforms. There will be differences often running into hundreds of pounds per year between the providers.
Just be aware that sometimes things are cheapest for a reason. HL do have a reputation for good customer support and a flashy modern website, whereas II is very much a DYOR and DIY platform with a clunky old website. Clunky old works for me. I drive an Omega :-)