If you decide to open a SIPP, I’d certainly recommend interactive investor. If you use this link to join, you’ll get 12 months fee free (saves you between £60 and £130 a year) and I get a £200 referral bonus. https://www.ii.co.uk/recommend-ii?ii_referrer=tkspbloppfji-5tjryty0hkyn
I’ve also used Hargreaves Lansdown and AJ Bell, but am in the process of moving everything to ii, as the fees work out cheaper