I am opening a pet shop shortly and want to sell mainly British products.
Does anyone know if this is legal or not here in Spain

What sort of products do you mean?  ‘Cos to be honest in most Ex pat communities (which if you’re loking at selling British goods then you’re looking to be in or near one) most places have English supermarkets which already sell those products anyway, so you’d have to offer something different to make it worth your while.

Certainly where we are (little north of Benidorm), in La Nucia, Altea, Calpe Albir etc, there are more pet shops than you can count….. defo not a shortage.

I am surprised you did not check these details before deciding to open! I agree with the last poster - there are already loads including the big boys….....

Good luck!