The USA has over 39 times as much land as the UK, and its population density is over seven times lower. Therefore the average distance that goods have to travel inside the country to reach consumers are far greater in the US than the UK.
Because of this, and the fact that the UK has a universal pricing system for postage (i.e. it costs the same to send an item within the country regardless of its final destination), the cost of shipping is lower and provides a greater incentive to buy and sell goods online.
It also makes it easier to provide a multichannel shopping experience. Small distances mean that it is easy to move stock to provide Click and Collect or Click and Reserve options, details of which are outlined in our How the Internet Can Save the High Street report.