Hi - this is probably obvious and won't be of much help but - if this is an essay you are writing, don't lose sight of the fact that just because a contemporary British film fails to represent anything that could be called a British national identity it doesn't necessarily mean that there is no such thing as a British national identity.
I personally believe that there still is such a thing, however, if you want a high mark for this then it would be best to argue that 'there isn't (but...)' because no matter how well you argue in favour of there being one, lecturers and teachers will still mark you down because none of them think there is.
Having recently finished a media degree, I know from experience that if you even dare to suggest to university lecturers that there are still British characteristics (or also English, Welsh, Scottish etc) they will view this as tantamount to racism... They only really want to hear British national identity discussed in the same breath as the BNP and football hooliganism and they only seem to view Britishness as a negative thing.
Generally speaking if you mention class, was there ever Britishness or just Englishness projected on to everyone else?, multicultural society, Americanisation, and fragmentation (as opposed to mass audiences) you'll probably be on the right track.
Sorry I can't think of another recent film to compare with Trainspotting, but you could also make some salient points about (tourist's idea of) Britain and Britishness as seen in Richard Curtis films and contrast that with the films you will be focusing on.