The big PI(and it is BIG!) at Number 15 Bath Road also has a 24 hr Costa Coffee. I booked the meal deal when I booked as an extra. 2 courses for evening meal plus a drink and then unlimited breakfast. I think you may be able to book it as an extra when you get there though. Other than that you will be able to eat in the restaurant from another menu. I know when I was there that there were quite a few people doing this and seemed very satisfied with what they were getting. The restaurant gets very busy! I would guess that if you were eating "off meal deal" that you would pay extra for your breakfast in the morning.
Presuming that you are going from Terminal 2 or 3 this is the one that you want. The other PI is right at the other end of Bath Road and is used mainly for Terminal 5.
Of course you can always go to the 3 Magpies for a meal! I think (although I am not sure) that is a good walk from the BIG PI but I am not an expert. Mick W introduced me to everything I needed to know about Heathrow and the free bus routes etc. :thumbup: