NW3,
Have a look at The UK Code Assigment Plan in the UK AIP ENR 1-6-2-4 on
www.ais.org.uk (free to register)
This, along with ORCAM (Originating Region Code Allocation Method) determines which series of codes apply to which route, depending on which airspace each flight will transit.
Each set of codes under ORCAM will have multiple users. For example, 0501-0521 might be for UK-France flights. They might also be used for Italy-Africa and German-Russia.....a/c on each route will never be in the same airspace.