I look at the fee being offset/paid for by the interest rate which they offer. The first £3,000 in cash (Inc reserved cash) gives a 5% rate, or £150 per year.
I always make sure I have this, either in my limit orders, and the rest as cash for investments that I want to make on the spot. Everything over that is either parked in treasury bonds or Sonia/cash bonds to make sure I get interest, which I sell if I need to top up the cash balance.
This way I’m simply using the fee to transfer money from the real world into my SIPP - ok you lose most of the interest on 3k, but it’s not a big deal, I get much less from my current account with that sort of money in it!