Welcome to the forum.
Wait for the avalanche of responses. Depends on a few things, your knowledge obviously (if you're intending to head across the channel at some point in the future), then you must have experience, VHF licence, plus relevant RYA power-boating courses, etc.
Back to the boat. Probably 5.85 minimum (image courtesty of Ribcraft) with 150hp on the back if that's your long-term goal. That would give you enough seating (how many people normally on board), engine power and fuel capacity. Obviously budget, where you're storing it (think of security) and even the towing capacity of your car/truck will determine what you're looking for.
Of course you can go smaller, but deep V hull and boat length are key to negotiating UK waters in the lumpy stuff, dry ride and comfort, especially if you're venturing away from the relative shelter of coastal waters if the weather changes.
The Ribcraft 4.8 has made it around the UK a few times, as has Ribquest 4.8, so that's as small as I would personally go.