We always recommend getting to the airport at least 3 hours before your flight. That way, you've got plenty of leeway in case things take longer than expected, and - even better - more time for Duty Free shopping and coffee-drinking. Winner.
It's also a good idea to check your airline's website and specific airport information to see what they advise. And - just to be extra safe - we recommend checking the gov.uk Air Travel site before you travel.