If you simply must have a ski the Futura 2 (
www.surfskis.com)is a good bet. Vince, the owner of Futura is a good guy and will steer you right. It is not a race ski but you will be able to paddle it. If you do race in it you'll probably do well against sea or recreational kayaks or even other surfski paddlers who cannot paddle their "faster" boats due to lack of seat time. Stability comes with time in the boat. For your first boat a vacuumed bagged fiberglass is a good bet for cost versus weight and not as fragile as a carbon boat. Believe me, the learning curve in a modern surfski will be humbling and the water in the UK isn't as temperate as Hawaii. A modern, fast surfski is about 21' long and 17" wide. It becomes stable when it is paddled with the correct stroke. When sitting still it is like balancing on a bicycle that isn't moving.
Go join the Yahoo surfski discussion group and make a few contacts. Try before you buy. Paddlers are a good lot and you'll find someone to take you under their wing. Very similar boat is the K1, Olympic kayak. The difference is that you sit "in" a K1 and "atop" a surfski. K1 paddlers can adapt quickly to a ski. So, look up a K1 coach at a university or club and get involved.
I took up paddling for fitness when I gave up on running and I wish I'd done it years earlier. I've evolved into paddling with an OC6 club(6 person Hawaiian outrigger) and have a new OC1, a Huki V1Z.
www.huki.com, The skis got sold but I enjoy the hell out of the OC1. Basically, an OC1 is kind of like a surfski but with an ama or outrigger. You use a canoe paddle vis-a-vis a kayak wing paddle. You can paddle it from day 1! Right now a team from the UK is competing in the World Sprints in Sacramento. So, look up an outrigger club in the UK. They are always looking for paddlers and have coaching and alot of fun races with many categories depending upon age, sex, experience. Before you invest look around.
FYI, Tom Selleck is or was a very good paddler. In the series he is paddling a Hayden (Australia) which was specially made to accomodate his stature. He uses a standard kayak paddle as the wing paddle was not invented yet.