Congratulations! I have been every year for the past three and love the UK--you'll have a blast. I think you should be able to swing that budget. I would highly, highly, highly recommend The Riverhouse Backpackers in Cardiff. It is inexpensive; it's a hostel, but I've found it to be more like a bed and breakfast except you share a room. I've stayed there both times in Cardiff. It is easy walking distance to the castle, the main hub of Cardiff, the train station and the bus station. I have a link somewhere if you'd like it. I honestly cannot speak more highly of it. I unfortunately have nothing on London accommodations--I usually stay with friends. And if you want to stick to your budget, I'd stay away from Forbidden Planet in London, as it is Geek Paradise.