We booked our Day 2 tests with Randox at the Radisson Blu near Heathrow. £50 after a £5 discount code. We took the tests about 8:45am and got our results less than 12 hours later. It's a bit more expensive than some of the tests that come through the mail, but we felt the quick result was worth the extra cost. There are testing facilities at Heathrow, but they were more expensive so we opted for the 10 minute drive to the Radisson.