The Generator is probably the cheapest place to stay in London - it's ok, but I'm not confident about the security of any gear you don't have locked up.
The New Dome Hotel is pretty good for the money. I've stayed there a few times and always been able to get a good kip. It's not that near to a tube station though, but it's fine for buses.
There's a very cheap place on one of the side roads opposite St. Pancras - run by Eastern Europeans, cheapest rooms are shared bathroom & shower - it's a pit and I wouldn't leave anything you want to keep in the rooms while you're out, but it's surprisingly quiet, clean enough, and, as I said, cheap; especially for somewhere so close to transport links. Can't remember the name of the place, soz, but it'll come up on Booking.com or LateRooms etc.
That's the thing to bear in mind - apart from luxury you'll have to pay more for good transport links... which may seem like a ball ache, but a few quid extra for a choice of tube/bus/train can save a 'kin fortune later. Oh yeah, and download the timetables of all transport that serves wherever you choose to your phone before you go, so you're not having to wait for it/them to d/l when you need 'em, and familiarise yourself with 'em because, and trust me on this, there's plenty that costs an arm and a leg in London and it's such a bastard of a feeling to realise that, if only you had realised in time, literally a few minutes ago, you could have got a bus but now you have to wait 2.5hrs for the night bus or get a cab...