Costs - probably they own the airfield and/or premises, so no rent, landing fees or hangerage to pay to a third party.
One thing most people will advise against, don't pay any more up front than you could afford to lose - or pay using the protection of a credit card if you have to. This is no reflection on Clacton, but applicable to any flying school - they can, and do, go bust.
Good luck with your training, I'm sure there will be somebody on here who has trained there and will be able to give their views on the club itself.