[i]Having an ISBN obliges you to send 5 copyright copies to the copyright libraries via an address in London. The British Library will also want one. So add the cost of 6 freebies to your cost-per-unit.[/i]

But it will have the advantage that your publication will be indexed in your National Bibliography and publications like British Books in Print. That in turn will give it the benefit of turning up in bibliographic searches and may add to it getting around.