There's a map of the Dethleffs dealerships here
Lowdham Leisure at Huddersfield have Dethleffs caravans. LeisureTimes at Stoke on Trent are also advertising Dethleffs, and we recently saw at least one Dethleffs in a forecourt at Caravan Sales at Garforth, on the A63 towards the A1 - somewhere we've not noticed before.
If you have a look on Caravan Finder you may find more dealers selling second-hand Dethleffs.