I was in UK London for about 11 days in 2012. I was told to use Tube for travelling as it is very convenient. We travelled from Heathrow to the central city via Tube. Once we reached central London, we got the Oysters card (which is preloaded value) in your card and travelled almost everywhere in London via Tube. You can top it up at any station over the counter which was very easy.
The Tube was mostly on time, well ventilated, clean and efficient. There were 1 or 2 times where the tube had some problem hence delayed but it was notified via announcement in the platform.
Lots of people travel using tube so I would highly recommend that you stay on the right (the slow lane) if you are not sure which direction to go to, etc. At times during certain peak hours, you can see lots of people are really rushing to get to their train, to switch train, etc. Just words of precaution, if you are female traveller or perhaps travelling late night, do avoid standing close to any group of people. There were one time we were at Bayswater station and there were a bunch of youngsters (early 20’s) who looked a bit tipsy and were making a little bit noise. They didn’t bother us but we just avoided standing near them. Anyway love the Tube!