Like, and possibly thanks to, Wimbledon, strawberries and cream are more or less synonymous with summer in Britain. The first few weeks of July, strawberries are in season and Wimbledon sells almost 200,000 portions during the tournament.
If you can't get clotted cream and don't want to make your own, you can use ordinary double (heavy) cream or whipping cream. It's up to you whether you whip it so it's thick, or leave it runny.