Eventing is specific in that you can only really aim for one major competition in about 4-6 weeks. Even if you focus on the short format events, you still need 10 days between two cross-country completions in regards to welfare. So the gaps in a calendar are a fixture here.
British competitions have indeed been largely reduced to just domestic riders (Britons and internationals residing there, mainly from Australia, New Zealand and Japan), plus the usual dose of Americans who make the trip. The amount of riders from the continent has gone down, and that's across the board (in particular Germany and Sweden, but also Ireland, France, Belgium, Netherlands, Spain).