I went through a very similar situation at my daughters wedding last year, and was really worried about seeing my ex again after 7 years as he was highly controlling and I knew my default position would be to feel inadequate and want to go hide somewhere.
The first meet was the church rehearsal the afternoon before the wedding, and typically he walked straight up to me and put a possessive hand on my arm, when he said hello!!
You know ... it was OK. I managed a very frozen "good afternoon" in response, and what the bridesmaids described as a very withering look at the offending hand! The main thing was that the worst was over. We had met again.
From that point forward, I ignored him!! .....and had a wonderful day. You really will be so busy with people wanting to talk to you (in America or not, you are the mother of the groom and people will be flocking to meet you). My darling daughter had arranged seating so that I was not even directly looking at my ex, let alone sitting next to him (benefit of a round top table!). I am sure your son and his bride to be would be very understanding if you tell them you are fine with it all (thats important!! - they don't need your angst right now) but is there any possibility of not sitting next to each other. Its worth asking. As you will be the only UK attendees, could you ask your future d-i-l if she might task a willing relative or two to introduce you around, as you won't know people and the couple will be busy??
Honestly, on the day, I think it will be far easier than you think to stay out of his space.....and thats all you have to do in order to have a great time. The key is in believing that you are OK. That divorce means you have taken away his power to upset your life.....and that means at this wedding, more than any other time. Truly, ex's only have the emotional power that we choose to still give them. Don't let the oast spoil this wonderful day.
x
P.S. ....and I would go to the UK celebration. You were obviously married for a long time and I am sure were regarded very fondly by many of ex's family. At this kind of event, they have an excuse to enjoy seeing you without anyone saying they are being disloyal to your ex. The same rules of just avoiding his space apply. And you can always leave early. I think you will regret not going for a long long time, if you miss it. And more importantly, your son and d-i-l will remember that you weren't there for them.