Question above re HL fees wasn't fully answered: funds / ETFs / trackers have a % annual fee, used to be called AMC / TER now called OCF / OCR. The OCF / OCR is the total costs, fees, charges that the fund provider/manager takes per year (fund unit price reflects all these costs). However, with some providers like HL there is an additional "platform fee", in the case of HL it's 0.45% per year for all funds. But the % platform fee does not apply to ETFs, individual shares.
So your total cost for a fund with HL is OCF + 0.45%. Whereas flat-fee platforms charge £20 a quarter, or £100 a year or so. Clearly when your fund investments are £50k or more, you would want the flat fee structure not the platform % fee to minimize costs.
It may be that HL offer additional investment/trading options, so if you want to invest/trade actively or you won't have much money in funds, the HL platform fee may seem reasonable/not relevant for you.
Though to complicate matters further HL do offer a "rebate" on some funds, e.g. half of OCF is returned to you (I assume the rebate is periodically used to buy you extra units, I don't know).