I can't answer specifically for the UK, but I can tell you what's worked for me: ask when you reach a decision point where you need to know the answer.
First, during the interview process, proceed as if it will work out and everybody's being reasonable. You can ask about this later, so why risk interviewers thinking "oh dear, this one's going to be too much work" or the like?
If your acceptance of the offer depends on the accommodation being made (you won't work there if they won't do this), ask as soon as is practical after you receive the offer. This is the same point in the process where you would bring up that two-week vacation you've already booked, or your need to leave early on Fridays because of Shabbat, or your questions about working from home occasionally because you're the primary caregiver for your elderly parents, or whatever. They already want you at this point; they'll be more open to discussing things than they would have been earlier.
If your acceptance does not depend on the accommodation (but having it would sure make things easier for you), then wait until you start. Especially if there are things that you can do to mitigate too, they'll see you doing what you can and (in my experience) be more inclined to do what they can. You'll have to get by without it for a bit longer because they won't have prepared, of course.
Here's an example of how that last approach went with my current position: I have some vision problems, which among things make me more susceptible than normal to problems caused by glare and lighting. I also tune my software environment in various ways (larger fonts, color settings, that sort of thing). On my first day I asked the people sitting near me if we could either close the blinds or rotate my desk to get glare off my monitor, which was no problem. A few days later my manager (who is not local) visited, and as we sat together at my computer he said "you could use a larger monitor, couldn't you?". I said yes that would help, and he promptly got the approvals to order me one. Had that not happened I would have waited a few weeks and then brought it up myself. A few weeks is long enough to start demonstrating value and get past all the "new setup" stuff.