Just to follow up on this, I took advantage of the free 6-months trial of ii’s SIPP. Only plan to do regular investments with ETFs anyway, so the only downside is that I don’t get to choose the purchase date each month. But you can change the amount the day before with no issues (or at least it was fine when I increased the investment amount, not tried to reduce it).
I’ve just been forced into switching my FT subscription to annual because it’s only a few days until my renewal. But if FT still isn’t supporting employer contributions in the SIPP this time next year, I’ll be downgrading to the standard plan and just use FT in maintenance mode going forward.