In general if you're trying to drive through a major city you're probably doing it wrong. Cities are far too densely populated for inefficient motor vehicles to be the primary means of transportation. You need to be using the tube or a bus instead.
Barcelona has 1.6 million vs Londons 8+ million. Also Barcelonas street layout was mostly designed in one go, whereas London’s street plan just kind of congealed over the centuries.
Like most UK rail, it’s anywhere between somewhat expensive and _outrageously_ expensive, depending on exactly when you buy the ticket and how. Looks like it cost me 9.50 last time I was there, which is not _horrific_ for an airport (airport transport generally being a bit of a ripoff), but the barely-faster Gatwick Express would have been far more.
(The UK rail system is a uniquely user-hostile one, in that it seems designed to fool you into paying more than you have to.)