London Visit

Hi all, its the girlfriends birthday in feb, i’m looking to taking her to that London, now i know quite a few of you visit London from time to time, does anyone know of a cheap hotel or something (im on a really tight budget at the minute but id like to take her away for a few days ? not premier inn as they want 200 quid for 3 nights when i try to book up, suggestions would be most welcome, ideally within walking distance of the main attractions? i have 0 idea of where to go as i’ve never been into London other than a trip to RAF Hendon.

Thanks in advance