Some London Escorts can only offer outcalls. This is usually down to one of two reasons. Personal preference or circumstances.
To be honest it is usually the second as many Escorts live with friends or family who may not be aware of what they do. Sometimes in London it is difficult to find a place to work from and expensive so they do outcalls until they have saved enough money.
Some Escort Agencies only do an outcall service so this can also be another reason that you will see Escorts displayed this way on the directory.