Where do people usually arrive when they travel to Britain by sea or air?

There is no single answer to that. Britain has many airports, so it depends on where people are travelling to exactly, as to which airport they will fly to, and also where they are coming from. London Heathrow would be the biggest airport, but there are many other airports that people fly direct to. People will get to the nearest airport to their final destination.

It is the same with the sea ports. There are many of them that people can travel to and there are many places that they would be travelling from, so there would be different options depending on those factors.