Some people I know on another forum sell once a year in January, put the proceeds in a money market or savings account, then have scheduled transfers to checking to recreate a paycheck-like situation.
I don't like to have that much money sitting around in cash, so what I do is just refill my checking from my savings when I need to, and refill my savings from my taxable investment account whenever it gets low. I usually do 3 months of expenses at a time. I could refill monthly but that feels too "fiddly" for me. Since I have non-portfolio income, that "3 months of expenses" can end up lasting six to nine months or more.
I refill whenever needed. The one concession I do make to timing and amounts is to look at my cash needs at the end of the year in light of my tax situation. Whatever LTCG I generate ends up reducing the amount of Roth conversions I can make, and I Roth convert up to a certain AGI each year. So I make sure I have enough cash on hand to get me through the end of the year, then do the Roth conversion. I wouldn't want to overshoot my AGI target if I needed more cash in December and had to generate more LTCG to get it.
Since I sell frequently and plan to keep doing so for the next thirty years, I don't worry about the short term market fluctuations. I expect I will win some and I will lose some and it will average out in the end.
With Vanguard mutual funds and no hurry to transfer the money, I pay $0 in transaction costs. It sounds like your UK pension situation has fees. I'd probably look into seeing if there is a way to get rid of those fees. In the US sometimes that means rolling an employer plan into an IRA; I'm not sure if there's something comparable across the pond.