Hi Chris. I tried to do a GBP deposit on Coinbase myself today and was disappointed to find that it is not possible. I emailed Coinbase support and much to my surprise they responded really quickly. They stated..

"We will start enabling GBP deposits/withdrawals progressively to all customers over the coming weeks. We expect all customers to have access by September 2018.

Please be assured that we are working hard to to ensure this new system is thoroughly tested before we launch. You’ll be notified as soon as it is available for your account."

So there you have it, all being well it should be good to go sometime in the next three weeks.