19 days after my 4th request for info I had a reply. . . I guess they are busy with the EASA stuff
If you email/fax a completed form SRG1199 they will send you a letter (paper or electronic, didn't ask) confirming your English proficiency.
There was no mention of a fee, and, given that it is in reality an omission to define your level in your licence page , on their part, I see no reason why there should be.
A licence verification letter, if you need one, is 43 quid these days.