Re: What do I have and what have I let myself in for?
Yes, do check the WCHA site out. There are lots of folks on there including pros that will help you.
If the varnish is not failing (cracked, crazed, blistered, peeling etc.) you could get by with a good cleaning with trisodium phosphate TSP and then sand it with 220 grit and revarnish, scuffing between coats. Use a good quality marine varnish like Epiphanes, Captains, etc. Don't use any polyurethanes. Don't use any car wash detergents or other things that contain silicone. This will inhibit your varnish job later.
I would chemically strip the old paint and failed varnish. I don't know what is available in the UK, but I use methylene chloride based chemical stripper (gels) and plenty of safety gear. Use plastic brushes, plastic scrapers, and plastic brillo pads and work with the grain to avoid damaging the wood. There is lots of information on stripping on the WCHA site.
I would not use bottom paint on your canoe. I would strip if necessary and revarnish the entire canoe.
The decks look like poor replacements to me, but I can't really tell from the photos. Someone on the WCHA site could probably ID this canoe for you. Although it is a distinct possibility that your canoe was built in UK or Europe, lots of Canadian boats did find their way across the pond too.
Good luck with your project.
"Wherever there is a channel for water, there is a road for the canoe. " - Thoreau