The problem is that anything with enough umph to have a bit of fun for 2 guys that weigh 230kg is going to be too much boat to singlehand.

You will singlehand a Hobie 15 ok but the two of you will be too much for it together.

An 18 footer will be the best compromise, the dart 18 is very underpowered and won't like those beaches! The Prindle 18 might be your best bet, not many about but they will be that sort of price. There were Pridle 18s at Bognor Regis SC I know.It might be worth a look in at Pevensey Bay SC to see what is for sale there