You can of course choose to be domiciled in the UK. You can of course only be domiciled in one country. If your home and heart is in the UK, then you have every right to be domiciled in the UK.
If for some reason you think you might leave a large estate and you want your children to inherit it, there may be tax advantages to settling your estate in the US, assuming your children are in the US.
The only time, either in the US or the UK, where a will would really become an issue, is if someone in your family would contest the will. It would have to be taken to probate court in the US or the UK and would not likely be invalidated in either case. They would attempt to figure out what the intent was.
If you have minor children involved, it may make sense to have the will done in whatever country where those you wish to have custody of the children reside. This would make sure that your wishes would be carried out in the event of your deaths. Other then that, most probate would be clear and straightforward and it only becomes an issue for the tax man.
If you have a large estate, then you probablly should already have lawyers and need them to figure out where the best tax advantage is for your to be domcilied (which would not be the UK).