Yes if you can get a train into a UK airport that's definitely the way, but even in the train-friendly UK that's often not feasible. For example Bristol airport has no railway. Heathrow is very inaccessible by train from the west. So there are unfortunately still plenty of reasons to drive to UK airports.
Bus to Bristol Temple Meads (railway station) used to be pretty good. Bristol now charge cars £8.50 (for 5mins) to use the drop zone. This means lots of cars dangerously trying to set-down/pick up passengers in the busy main roads outside the airport. The government should step in; passengers should have a drop zone they can safely walk into the terminal from.