I don't want to socialise with my colleagues but I feel guilty saying no?

lol I know what you mean, cos these are work people not friends, recently I went along to an after work drink thing as a guy I sat next to was leaving, I stayed long enough to buy him a drink, say hello to everyone then said I needed to get some cigarettes (I don't smoke) and nipped off! So I made an appearance then went home everyone was happy!