There are numerous A2 colour poster printing providers in the UK. Although not all provide specific materials and finishes for outdoor use.
There are several things to consider when designing for outdoor poster use.
Firstly the substrate which needs to be robust enough to withstand the elements, surfaces and the environment in which the poster will be used. The weight of the various papers available need to be coupled with the finish your design is looking to present. From a matt, gloss or satin look there are many stocks to choose from but only a few that are really fit for purpose.
Secondly the additional protection over laminates can offer outdoor posters should not be underestimated. Lamination encapsulates the paper substrate therefore adding weight, thickness and protection from the elements.
It is also worth mentioning at this point the option that vinyl banners can offer if you are looking for year round and long term use. Although not as aesthetically pleasing or as suited to full colour photographic images vinyl posters are excellent when large text or signage is required.
For more information on poster printing visit printmyposter where you will find alternatives from A2 colour poster printing and A1 posters printing.