Good for you, OP! A very thoughtful thing to do. And to be honest, a good use of a four hour connection... the lounges aren't *that* good
You might also want to pick up a tin of tea from Harrods, airside. Fairly decent tea and the containers some come in look quite nice.
It's a personal thing but I don't understand why people are saying just get Twinings.
One thing that's occurred to me, that others will know much, much better than me: if transferring airside T5-T3, I'm not sure how easy it is to then get landside once you're in the departures area? It may be easier to get out at T5, check out the different M&S's, then get to T3 to get back airside.