Always remember to check first-class fares as well as standard, as for some journeys (especially weekend ones booked not very far in advance) they can be either within a few quid or sometimes actually cheaper.
Same thing is true on planes, especially at this time of year when lots of people are travelling for family reasons (so demand for economy seats is high) but very few people travelling on business (so demand for business-class seats is low). A couple of years ago I flew to Sicily for xmas and business class was £30 cheaper than economy.