I'm thinking for the 4 of us with luggage that a van would be better than a sedan or station wagon (estate?). Is Uber or FreeNow an option for that? From what I recall of the black taxis, I think the 4th person would have to sit in the "jump" seat facing backwards. Which is fine for short trips, but I'm thinking not that comfy for the ride from Heathrow to Canary Wharf.