Don't buy a monthly travel card. If you end up in halls there will always be a bus that can take you into central London - either the 9/10/52 from Albert Hall, or the 14 from Evelyn Gardens.
The tube doesn't run after midnight, whereas the bus routes aforementioned are 24 hours, and are only 5 minutes slower, if that. You save much much more money paying 80p each way on a night out than having a £50 tube pass which tbh won't be worth it if you don't use the tube everyday.