An RV-7/8 would also meet your need for speed, the construction quality can be more easily checked and it can be flown on short grass runways. Quite a few RVs have been successfully bought onto UK permits.
Some other thoughts:
Regular 600nm trips will need IFR capability unless you are flexible on time, so not suited to permit aircraft.
The exchange rate, VAT and shipping/ferrying costs will bring the US price up to UK levels with the extra hassle of registering, inspecting and gaining permit approval. You would have a wider selection to choose from though.