This is probably a lot harder than it sounds but I'll go with the simpler stuff. I apologise in advance if this sounds preachy but here goes.
Cut out all the alcohol or else limit it to drinking in moderation one or two times a week only. During academic holidays, don't drink any alcohol at all and detox.
Cut out all added sugar and salt, buy fresh vegetables from the supermarket (which in reality is nearly dirt cheap compared to anything else they sell) and buy bulk bags of rice and pasta which are also cheap for the calories on offer.
Exercise vigorously for at least 60 minutes a week though 90 minutes would be better. I'm sure nearly all Uni's in the UK have access to a gym. Go there and use the rowing machine- get someone to coach you on how to use it properly. Row 5000 metres each time (should be done in under 30 minutes- you're all young and fit so I'm sure 20 minutes is actually totally realistic). That's 3 times per week and your recommended exercise quota is done.
Totally avoid buying highly processed foods such as biscuits or other snack type foods. Buy fruit instead and snack on that.