I recently got the XXV while looking for a short, agile gun for low vis. conditions. I am v. pleased with it so far, it's a great gun. However, I don't think I'd recommend it as a first gun (although sounds like you've used others before, so perhaps this isn't like your first). I don't think it would be a huge mistake though. I'm not sure if I can articulate why I feel that, lots of little things but nothing of great consequence; on the other hand the Excaliber 2000 seems like it would be good beginner gun (simple, straightforward, tough). HF2 also looks like a good gun too and I am keen to hear more about it. In many ways though, it seems like a heavier/stronger version of the XXV.
Re. the length is usually determined by how far you can see & how far away the fish are. I would suggest an 82cm gun for the UK (or perhaps a 90cm XXV, light, slim & short spear), 75-90cm will generally work ok for the UK. For the med, seems like 90/100cm is a popular size. Re. length for the UK, I think of it something like this:
Cornwall - Devon - Dorset - Sussex - Kent
90cm ---- 90/82----82/75----75/70--------
But sometimes 90 is good for Dorset & 75 for Cornwall!
