There's no such thing as a "licence to do an area"
Sometimes one company will have a contract that covers a group of buildings (eg a warden-controlled retirement complex) but normal houses are fair game for anyone.
If you live in Scotland you do need a licence to window clean, these are obtained from your local council and cost less than £100 per year. You may run into difficulties getting one if you have a criminal record, but otherwise I think the application process is straightforward.
There is enough work for everyone, however its usually considered bad form to try and undercut someone's prices and take their customers. Also, that is asking for trouble in my opinion.
Better to concentrate on houses that clearly need a window cleaner, they are usually over-the-moon to see you and you dont have to convince them to swap-over.
-Philip