London: Taxi or Tube?

I would go with the tube, faster and less expensive then a taxi.

Definitely get travelcards. They qualify you for 2-for-1 offers. See here: www.daysoutguide.co.uk.

Are you going to see anything in the morning of day 3 before going to Kings Cross? If so, either a peak or off-peak card for that day might still be worthwhile, especially if you use it to get to Kings Cross.

We always use the tube as it's cheaper and usually faster. Be advised however there is alot of walking in the metro stations (making connections) and truly amazingly long steep escalators. For everyone but Grandma this is probably not an issue. I'd say her level of fitness and mobility are the drivers here. My mother did ok when she was in her late 60's as long as we took it slow and I had a firm gripe on her on the escalators. There are elevators but talk about hard to find...

Grandma will be 91 , yes you read correctly, ninety-one, but is in incredible shape. She still teaches an aerobics and exercise class, does Tai Chi and yoga, walks, drives, refuses to pre-board airplanes as that is for people that are old or have mobility problems, and she insist on pulling her own bags. Moreover, she is a prolific artist in numerous media and will produce numerous water color paintings in our three weeks in Europe. Our bigger problem is getting her to slow enough for our kids to absorb what they are seeing. The biggest advantage to the taxi would be keeping us all together, my biggest concern on a crowded tube platform.

I agree with the others that the Tube is best. But you need to have a plan in case you get separated. When I took my parents to England for their 50th wedding anniversary, we were in the process of boarding the train when the doors closed, leaving me on the train and them at the station! I tried frantically to signal to them to stay there and I would double back. When I arrived back at the station, they were there waiting, after a helpful local dissuaded them from simply taking the next train. (They had no idea of where we were going, where our hotel was, etc.!) Ever since then, we have a formal plan on what to do if we're separated. With a group your size and configuration, you need a plan like that.

On a London RS tour a few years ago our plan was to meet at the next Tube stop if our group got separated. Everyone in your group should have their own ID (probably their passport) on them and the name/address of their hotel. I pick up a business card at my hotel when I begin my stay and carry it with me. This card can be helpful when hiring taxis, too. Show the driver the card and they'll know where to take you. Your plan sounds good.

Do take (& experience) the tube from Heathrow to your hotel. Take it to the British Library, definitely.

But since you will be there for just 48hrs, keep in mind that not only can the stations be large, the walk to/fro can be relatively long-ish and tiring, no matter how fit you are--and seats may not be available on the trains.
For a little bit more $(maybe not with 5 people!?) but a savings in time and effort, your family of 5 should experience the wonderful, fun taxi cabs and views they offer (above ground).
I would consider taking the bus back to the airport from Belgrave Rd area (Victoria Station). It's more comfortable and you can see more, though will take more time. You wouldn't need a train card on this day.
Remember, your mom will get a discount at the Tower because of her age!

I led several senior ladys, ages 70 to 80s around London, we did fine using the tube. At the end of the day, they were worn out so we took a taxi back to the hotel.

The tube gets you around quickly and is much cheaper than a taxi. I was impressed that everyone seemed willing to surrender their seats to the older women. Mind the gap.

Just back from London. Taxis, in general, will be more expensive. The Tube is quick and efficient, and compared to taxis, less expensive. I will add, though, that if you want to "see" the city (dont know if youve been before) in your meanderings, consider a bus trip here or there; they should be covered by your travel or Oyster card.

I don't think anyone mentioned busses. If you have a travel card or an Oyster, you can use the bus system. We are frequent visitors to London and find the bus is, many times, more convenient than the underground. The busses are easy to use, the people at the stops are willing to help you find the right one and the view is better than the tube. We rarely use taxis. Even with luggage the bus is just as easy and a lot cheaper. A bus map for central London is available at most underground stations.

Was in London last year for a short trip as well and I have to go against the grain a little here... While a pound-to-pound comparison may reveal that the Tube is cheaper, when I was there we had frequent delays on the Circle Line. Considering that you are there for only 48hours and time is money, your time might be better spent not waiting on a tube platform or making the hike just to get to the platform - at least with a cab, you are able to see the city a bit. If you were there longer or traveling solo/couple I'd agree w/ taking the tube.

RE: Oyster Cards - it was a bit of a pain to return to get the deposit back (long line). For your short trip, if you decide to go with the tube, I'd suggest just getting the TravelCards.

We have been to London more than 40 times. We only had to use a taxi one time- on 7-7 to get to the airport after the bombing. London traffic can be unbelivelable, and while the taxi drivers are the best in the world, they can't work miracles. The Tube and buses are the best way around town. And to tell the truth, much of the city can be accessed best just by walking! Consider taking a boat trip down the Thames- you may get a discount with your Tube pass. It is well worth it - and spend a little time in Greenwich, too.