LHR T5 is very well-served by restaurants, and you'll be able to have a good meal (with alcohol, if you wish) for less than the price of entry to a mediocre lounge.
If you think you'll be hungry on the flight you could also get a very good sandwich, crisps (chips), drink and dessert for around £8 from Pret or Eat and take them with you. There is no guarantee that the buy-on-board trolley will have what you want - or anything at all - by the time it gets to you.