We will be landing a Heathrow in 20 days and will need to get to Kings Cross Station to catch a train to York. We are senior citizens and I don't feel like taking Tube with all our luggage (we are traveling light...but still...). What is your advice of (1) taxi picked up at airport, or (2) a pre-arranged car service (and which one?)? We haven't been to London in 26 years so I need your help! (In my defense, I have travelled way too many times to Paris in those 26 years and have total confidence in the official cabs from Orly!). But London is a whole new world for me!! Many thanks for any advice!
I am so sorry to not be answering your specific question, BUT, we ("young" seniors) took the Picadilly tube to Kings Cross and it couldn't have been easier. We rolled our suitcases onto the tube and sat until we got to Kings Cross where we got off and caught our train to York. This was our first trip to England. The tube was comfortable and not at all crowded.
I hope you also get a response to the question you asked, but I wanted to share that we had no trouble at all with the tube.
I also can't answer your question, because we also take the Picadilly line to our hotel, which is close to Kings Cross. And we are mid 70s, with suitcases and day bags. And I walk with a cane. Finding a seat is easy, since LHR is the starting point. According to Tfl (Transport for London) both the Heathrow and Kings Cross stations have step free access.
If you take a taxi from the queue it will be an official licensed taxi and the fare will be metered. If you book a pre-arranged car service it will be a fixed price which is typically less expensive. Two of the car services frequently recommended on this website are Blackberry Cars and Just Airports. I have no personal experience with either of these two services as I almost always take the tube.