Someone earlier was talking about the "Minuteman" group probably not going because of their muskets. To be honest, that really shouldn't be a problem. I've traveled to England several times with a large group of Civil War period reenactors, to support some of the ACW reenactment groups over there. Yes, there are several thousand folks in Great Britain who reenact battles from the American Civil War. Go figure...
Anyway, the smoothbore muskets that the Minuteman group uses are classified as shotguns in England, and can be brought in with proper planning and permits. When we traveled we obtained temporary permits and honestly had absolutely NO problems with the police, customs, etc.
The only thing that we couldn't bring in was the black powder blank cartridges and that was because of the airline, and not the British laws. We simply contracted with the local reenactment group to provide us with the blanks we needed.
If the Patriots' Minuteman group wanted to attend, I really can't see any reason why they wouldn't be allowed to.