where in the UK?

There is a substantial difference between the north and south UK.

I lived and sailed in Campbeltown Scotland for a few years. all 2 weeks of summer were glorious sailing, but after day 15.. winter hits hard

I would personally recommend a catamaran designed with the North sea winds in mind (Dart or similar) before you jump into a 20 foot beachcat that may (will) get overpowered in above 25knots. a 20' cat can be sailed in 25+ but really requires great skill and hopefully a fair amount of experience depowering such a powerful sail plan.

I have crewed on a dart 18 and dart 20 in pretty extreme conditions (through squall lines up to 45 mph) and both did very well (and did not flip)

There are a few people from the UK on this forum, hopefully they can help steer you towards a great first beach cat.

good luck