8-9hr layover London Heathrow (Terminal 5). Where to go to?

I have two 8-9hr layovers at London Heathrow coming up from next weekend, always Terminal 5, EU passport holder. Any suggestions what to do? I've just been to London this year and have no further ideas for that. As I 'collect' UNESCO sites I'm considering going to Kew Gardens on one day for 2 hours max to be sure I make it through security. I'm only traveling with handluggage, a 40l Patagonia Black Hole, but it's comfortable enough on my back for a few hours.