Your uncle will have a great time in the UK (and we need the money from visitors too
). Bike shows in summer are a bit rare, come to think of it, summers are a bit rare in the UK too!!! and we tend to have the big shows in the spring or autumn. If he has a car he could travel about 170 miles from the National Motorcycle Museum in Birmingham down to the South of England to Beaulieu Motor Museum in Hampshire, loads of bikes and historic racing vehicles. In the same sort of area is Goodwood home of the 'Festival of Speed'.
Links
http://www.beaulieu.co.uk/beaulieu-events
http://www.goodwood.co.uk/whatson/whatson.aspx
There are probably lots more.
CB