congrats cjga! your story is not so different from mine, so here's some reassurance:
1) we got married at the registry office in oxford. the cheapest room allowed only 2 witnesses, the next was for like, 45. we ended up paying for the bigger room (which was hilariously empty) because at the very least, my partner wanted his parents+brother there. it worked out that my aunt and uncle were there too (5 attendees total)
2) the formal witnesses sign the marriage certificate, and that's just 2 people. (in our case, it was partner's mom+my aunt)
3) do what you like afterwards! my partner is big into baking, so had made a couple of tarts ahead of time, and we all went back to our flat for tarts and prosecco (provided by the relatives). we went out to a nice dinner that night (his parents paid). i got my visa 3 days later, in person.
4) a few months later we had a wedding ceremony which was much more real to us, and a reception, with about 75 guests. what was great about this is that we could have a friend act as celebrant (which is much harder to do in the UK than USA!) and choose our own ceremony, rather than the rigid requirements by the UK state. We kept things pretty simple -- the ceremony was at 2, we only did afternoon tea for the reception, everyone out at 6. and then did a pub crawl with all the young attendees that night, starting at 9. It was absolutely the right decision for us to divide things up -- it took a lot of pressure off, not having to worry about the visa, and no one seemed to mind the legal bits had been done already.
So good luck with your plans! It'll be fine, and happy, no matter what you choose to do!