Was that frost forecast? It seems to depend on which forecast you saw. Early May in Kent it isn't surprising to get a frost, but rare to get one as thorough-going as this. If you had tomato plants out (a neighbour did) you'll probably have lost them. The potatoes, however, will recover and be very little affected.
Until late May, check the weather forecasts and be ready with sacking, net curtains or whatever to give things a bit of protection.