Difficult one Bookdreamer.
I too have son and DiL in America (their wedding was there too) and I feel I don't want to miss any opportunity to see them. But I can understand that the presence of ex and his family might seem too much to bear.
If you don't go to the reception in England, it might spoil things for your DS and DiL and in the future they may always feel 'Well we cant ask Mum because Dad will be there' and vice versa. There will be other family occasions, birth (hopefully) of DGCs etc.
On the other hand if you do go, you will come across as brave, caring and generous, and you will feel confident and strong
Why not take a rain-check? Say yes for now, and then if you feel really uncomfortable at the wedding, make up some reason for not going to the British reception, or just tell the truth.
I hope you enjoy the wedding.
Americans don't wear hats, so if you wear one, or a fascinator, you will wow everyone 