Romantic evenings in London? No restaurants please.?

My boyfriend and I have been dating for quite a while and are running out of cheap date ideas, which seems stupid because we live in London, there should be plenty to do! We're both poor students, so that does limit things quite a bit, as does our picky eating. Any ideas besides dinner and a movie or walks by the eye/parliament/trafalger? I'm dying to surprise him with something outside the house, but I fear our lack of income may be too restricting.