What type of glue/adhesive shall I use to collage my painted door?

Hello. I have a black glossily varnished door which I am planning on sanding down as much as I can before putting together a collage on it. I am going to use various types of paper and other materials and wondered what type of glue I should use and then whether or not I should use some sort of 'finisher' as well (presumably a type of spray/paint to seal everything in place and protect it even more). I am based in the UK and would really appreciate it if someone could give me some named products that I could easily find in any B&Q or Wilkinsons or somewhere. If anyone has any tips as to how I could go about doing this I'd really appreciate it. Cheers.