FWIW, we do timetable English and maths resits in this way for sixth formers who need to resit them- essentially they do it in an A-level option blocks. It does impact on the students who do resits- they can't do certain courses because of the time commitment, and also miss out on the chance to do things like DofE and EPQ, which isn't great for them. It's not quite 4 hours a week though- A-level students get 7 hours per fortnight for their subject.
Our pass rates for resits in both subjects are above national average, by something like 10% in English (which sounds good but is still significantly less than 50%)- so it does seem like it does help.
Normally we have one class of each, but had a bit of an issue this year as we started off needing two maths classes, which we then had to find staffing for! I believe now November exam results are out, the plan is to compress the two classes down to one. My understanding is that for the coming September there will only be one class of maths resit, and we'll potentially be turning away more students (which isn't ideal as there is a shortage of post-16 places locally).
On the one hand, I do think that for many students who struggled to pass first time round, if they only get an hour or two a week when resitting, they probably aren't going to pass, but on the other hand, I do see how it's really difficult to timetable. I know that the nearest college which does T-levels doesn't allow any resits alongside T-levels as it's too difficult to manage alongside work placements.
Realistically, the funding should probably be going into smaller class sizes at KS4, in order to help more students pass first time around.