With you located in the UK I would not hesitate to recommend a Dart 18. Should fit the budget with no problems.
Its good for soloing and works well with two, will carry three in a pinch but you won't be going anywhere fast with your crew-weight.

I don't see why a Dart 18 wouldn't like beaches? With no daggerboards or boom to worry about it is an excellent all-rounder. OK, its no where near modern, but its a good and sturdy boat all the same. As you get more experienced you may find yourself wanting more power, then you could look into a F-18 cat, but most won't right solo when you REALLY need them to, and that can make the difference between a tall tale at the bar and a headstone, if you get what I mean!

Besides there's loads of 'em around in the UK and (used) parts are readily available there.
Personally I sail a Nacra 5.2 which would also seem to be in the ball park, has a little more volume than a Dart 18, but with your weight it should also work well (has daggerboards though).