The nice thing about going to the Tate Modern on such a layover as this is that there is green space and a garden near there to spend a bit of time outside since that might be nice after an overnight flight. If you go to a matinee, I would book online in advance so as to not waste time getting the ticket the day of or risk being disappointed. Frankly. I would worry about staying awake for a matinee after an overnight flight and lunch but you may have more fortitude than I do. Depending on how early your flight gets in, you could grab a day room at a Heathrow hotel and grab a couple of hours sleep first. Or maybe go to a spa for a massage and a quick nap before setting out?