Hi Neil -
I won't go in for agency bashing, or recommend a particular brand, but as someone who has done both (drysuit and UK wetsuit) and who used to regularly act as an open water lifeguard for the RLSS at youth events, I will support storker and the others who say that the risks of hypothermia are real, and insidious. usually the last person to know they are being affected is the person themselves, when they start the uncontrolled shivering and get the loss of co-ordination and motor control that comes with it.
I would say that if you are in the diving game for the along haul, and not just holiday diving in warm water, then waiting a little while and investing in a good made to measure dry suit will pay you dividends and you will wonder how or why you ever dived differently. I have come across a few dive shops in the UK who run courses but don't have a plentiful supply of drysuits for students and who regularly put students in wetsuits, but usually they are targeting divers who are going to go on and dive in warm waters mainly. I belong to three different clubs in the UK, with somewhere around 120 or 130 active divers between them, and I can say I have never seen a club member diving wet, or even a semi dry. We have had a couple of club "guests" turn up and do so, but i they join the club then they have all gone into a DS very quickly.
Edward has also suggested that drysuit training can be found elsewhere than dive shops - there are clubs of various flavours, who have their own instructors and can do drysuit training in house and at your leisure. You may even find one that can get you in a loaned drysuit to train you, and then you can get a feel for what they are like, maybe try different types, and choose which sort will suit you best on how you get on with them. I gave up diving for many years, then when I came back that i what I did.
I re-qualified in mainly warm water, then joined a couple of local UK clubs and dived with them and learnt drysuits that way. They are not rocket science, but there are some potential problems and techniques you do need to know about and understand in order to make them safe, but you do not need a shop bought course and a card to get the knowledge safely. I'm not sure where in Scotland you are, but try looking at the BSAC and SSI websites and searching for local clubs, and try your local sports section of newspapers and local council to see who uses their facilities and pools. You might be surprised to find you have a very local club you just don't know about.
Best wishes and safe diving - and BTW thanks for the offer of the Scotch (on the other thread) I might just take you up on it. - P