[pukkabruv]

ive stayed at the cumberland, nice hotel, but yeah, far from a budget place.. YHA have hostels all over London, think the best priced one is over st.pauls way, also worth checking travelodge (kind of a motel style hotel chain, but they do offers of £19 per room per night, so worth checking)
make sure you go to carnival at the end of august, SW4 is on the same weekend, all dayer, line ups not too shabby.
how you getting from london to spain? think they still do train tickets that last for a month or summin, hop on hop off kinda thing. www.seat61.com is a great site fo all train travel information, all over the world