Vet UK, which I've used regular have it for £29.23 and you need a prescription.
Maybe prices have gone down for Optimmune, it is over a year ago now. My vets in Epsom charge £20 for a script, so it still would have cost £50 plus p&p per month, they won't do repeats, which wasn't much cheaper. Therefore this other company 'seemed' a good deal not requiring the script. VetUK are good, I've used them many times

Tali just has artificial tears now which is dirt cheap

It's such a minefield with medicines, isn't it?