Well, you're not going to like the answer, because it does seem to boil down to: Find a club.
From what I can gather, the exams are done by RSGB so you'll need to schedule an examination date at one of the clubs and go there to take the exam. There's a list of examination locations here: https://thersgb.org/services/coursefinder/
From what I can see, you don't need to actually go to a course, if you can learn everything on your own or if you want to take an online course. You just need to appear for the exam.
The exams have two parts:
The practical part is very easy (or at least that's how it looks to me), you don't have to do it at the same time as the theoretical part and the results are valid for a year. We already have a question about that here: UK Foundation - initial practical assessment, what to expect? It appears that you don't actually have to go to a club to do it, you can do it from assessor's location. There's a practical part for the Foundation license and there's a practical part for the Intermediate license, but you can do both at the same time, if you want.
Then there's the written part as well, for which you'll need the practical assessment certificate. From what I can see, you'll need to appear for that part at an examination center.
If you can't move for some reason, perhaps it would be a good idea to contact RSBG and see what they can tell you about that. On this page, there's the telephone number of their examinations department. Perhaps it's possible to come to some sort of an arrangement, if you're immobile.