I'm group 6, and have managed to book through that link! So massive thanks to OP for posting.
I'd tried several times before, but was not eligible, so something has changed - either they've added group 6, or my GP has flagged me up in some way to make me eligible.
My DH (same age/same doctor/same area) tried - he's not group 6, and it rightly came back as not eligible for him, so it must be based on group 6 eligibility.