Most banks will have them. Banks will take them over the counter from business customers in their usual takings, but will outsort them so they are not handed out to customers. When the banks have "collected" a certain quantity they will remit them to their cash centres for exchange for B of E notes.
So just try a few banks, but it will be hit or miss whether any have them. The nearer you are to the Scottish border, the more likely the banks will have them.