You can cool your heels for 8 hours in T1 (as number_6 notes, it's good but not as good as T4) or, my choice: go through immigration, park your bags at Left Luggage (£5 per bag, in the arrivals hall) and have a day in London via Heathrow Express. It's not terribly cheap--it'll cost you about $50 per person for the left luggage and train fare alone--but at least you'll have some fun and a nice lunch.