How do I get drunk enough for 5 hours (cheaply)

Hello u lot I’m coming up to my 20th bday (scary) I have £300 and am at uni, ive just finished my second year I may get £50 for bday from family if I’m lucky however it’s most likely gonna be a pub trip daily till I leave on the 30th. Any brits or the rest of u have any idea how I can get smashed on a budget. Cheers you lot.