Hi there, yeah sure, we can exchange experiences. If you haven't yet been to London, I can only tell you that it is HUGE! I would plan on spening more time in London than in Paris, which is relatively small in comparison. I would also recommend that you consider purchasing a London Card which can be used for reduced/free entry to tons of venues. The city tours are really interesting, especially the City of London tour, which covers a lot of London history. Piccadilly Circus and Trafalgar Square are a must. Also, you might want to visit an outdoor market on a Saturday. The London Underground has a comprehensive network and where it might fall short, you can take a bus. If you're flying into Heathrow, then take the Picadilly Line. You can buy a public transport pass after you decide how many zones you'll need. Yeah, that must be tough with the poor exchange rate, but it's just a fact of life. Consider cooking in the hostel kitchen. Indian takeaway is relatively cheap.
Good luck!