You are paying extra for the brand name. Viagra is nothing more or less that sildenafil citrate. Once Pfizer's patent expired anyone could legally manufacture and market sildenafil.
I also use Asda and Lloyds and Superdrug - l am normally issued with 'generic' (i.e., non Pfizer made) sildenafil but sometimes due to supply shortages l am given branded Viagra.
Have spotted no discernable difference between the two.