Lloyd's pharmacies (including those in Sainsburys) will sell a different OTC branded sidenafil, as well as Viagra Connect ... Aronix.
It works out slightly cheaper than Viagra, and both are OTC.
In agreement with other posters, and to avoid arguments over "splitting hairs" : it isn't strictly speaking a 'generic', (the pharmacist can't substitute any suppliers version, only that particular brand licensed for OTC sale, whereas for a true generic,on prescription, the pharmacist can dispense any version (be that Viagra, Aronix, a different brand OR non-branded)