Each has their own benefits. Most have a trading fee of about a tenner, though Charles Stanley allows you to trade FUNDS for free.
Most charge a holding fee for funds. AJ Bell charges 0.2% pa, and Charles Stanley 0.25%.
Hargreaves Lansdowne charges 0.45% (I think) for funds but holds shares and trusts (but not funds) for free. Their service and website are probably the best though.
TD Direct I haven’t used but I think (being Dutch) they have access to more european markets than most. They’ll also transfer in certificated stocks for a fiver, which ain’t bad!
I use Charles Stanley for funds, AJ Bell for speculative trading and SIPPs, and HL for everything else. I have ISAs in each. With SIPPs, don’t just check the holding/trading fees, but the exit fees too.
You may find that some funds are cheaper to buy and run through these “supermarkets” than directly from the asset manager.