When me and my friends went to London recently we stayed at the Howard Winchester Hotel literally about a 2 minute walk from kings cross station. It was about Ł30 if I remember correctly. You got a mini full english for breakfast too! The rooms were quite small but they were good because they had a little en-suite and they had large rooms so if youre going with a lot of people you can all stay in one room if you'd like to. It wasnt exactly the Ritz but it was clean and comfortable enough for us (Including one of my friends who usually moans about everything).