Another vote for skipping the semi-dry and going for a drysuit. Seaskin do excellent suits - you can get a made to measure suit for £500. You can add options such as pockets, zip cover, neoprene over-cuffs etc - mine came in at around £600.
A proper semi-dry won't be cheap. There are many wetsuits marketed as semi-dry that are really just wetsuits with Glideskin cuff and ankle seals; they will still let quite a bit of water through though. For a true semi-dry, the cost isn't going to be much less than a decent drysuit.
Some sites in the UK get a bit chilly at depth. When I did my CCR course in Vobster Quay, I was sweating like a fat lass in a disco when I got into my suit. The water was quite warm at the surface so I wore my thin gloves. For one dive, we had to swim at a steady 20 metres for 20 minutes to measure our O2 consumption rate, and by the end of it, my hands were almost dropping off until we got back above the thermocline. Dorothea in North Wales is very cold all year round.