I will add that ordering one of the paid-for meals on BA (only available from London) can add a bit of change and fun to the Y experience. Wheter the price is worth it only you can say! They are, after all, just reheated ready meals after a fashion.

Definitely have a bottle of water with you, and snacks that don't leave you dehydrated (raid the shops in the terminal if no time to organise before). You can bring your own amenity kit but the only thing I want during or after a flight is brushing my teeth...

I do sleep anytime anywhere, most of my travel (including 12h+ flights) is in economy and have only had status this year. I have however used lounges courtesy of priority pass/dragon pass and find them useful for smaller airports like Luton/Stansted. LHR has a lot of seats and food options, lounges can be nice but there are better options.