Ive done two very quick and dirty tests for myself.
Items like titebond and 3m 4200 stuff can be found in the UK but its pretty expensive.
Anyway tested two items recently,
Evostik Serious Stuff

- I am going to use this for aluminium onto plywood bonding. I did a test with some 0.9mm alu and some bog standard 12mm plywood. I didn't prep the surfaces, just put on a decent amount of adhesive then pushed them together, didnt clamp and left it for 24 hours. Came back and I couldn't physically pull them apart. After eventually getting one edge up with pliers it still was rock solid. I also left both items in a bucket of water for 6 hours and came back. They were still well bonded. At about £5 a tube most places I am gonna use this for bonding my sides on (well floating them on).
Evostik Waterproof Wood Glue

Basically, the UK's version of Titebond 2. Ive tested it with wood on wood, clamped two 2x2's together and bonded. After 24 hours couldnt separate them. In the end, put in a vice and hit with hammer, the second 2x2 actually snapped clean in half lengthways

Ive seen titebond in the UK, but for ease of use I think ill be using both these glues for most of my work. As most can be found practically anywhere.
I may still try and find some 3m 4200 marine sealant for the alu edging strips.
If anyone else has any UK glue or adhesive finds, please add! This is meant to be the start of a nice list!
Oh and I dont work for bostik lol!