Most airport minicab services from London to Heathrow are fine. The only bad experience I ever had was when my in-laws flew in from America for the winters last year. I had arranged for a limo several days ahead to make sure that they were comfortable when they left for the Heathrow airport. My wife had a lot of luggage and her grandmother was with her. Not only did the limo driver deny carrying the luggage, he also got angry when the 86 year old lady took her time getting out of the car! I will never ever use that service or suggest their name to anyone.
In panic, I called London Radio Cars and they arrived promptly and hustled us to the airport just in time for the flight! That is how reliable and highly professional these chaps are --- the reason why they are on my speed dial!