It depends what sort of flats you are looking for. If it is a big house split into a couple of flats then often you might get a garage or backgarden/shed thing
Im not sure if the bigger flats will have areas to accomadate windsurfing equipment. Some may have some garages but Im not really sure. Options are to get a van and leave kit in and good locks on it, or the other option is to leave kit at your club.
Queen Mary reservoir is probably one of the nearest places to sail, or draycote water.
The NW of London is only 1.5 hours away from Grafham water up the A1 also.