I have found that NHS issue the certificates containing your middle name as it is a British custom to write letters with Last name only.
In my case I'm not a British born so my last name is actually two words, but my GP registers the first word as my middle name, so I called my GP and they were happy to amend it so that my last name has two words.
They had to force sync their database to NHS, but that is also done overnight anyway, I got it printed correctly.
I suppose it won't be a problem anyway, but not being able to do an online checkin also means you get worse seats.