Hi Just Jo --
Sounds like a good business opportunity. I would definitely focus on the geographical areas that have a large British expat population... especially where little to no Spanish is spoken.
I had a interesting conversation once on the train from Gandía to Valencia (in the Valencian region) with a British woman who had been living in Spain (concretely Oliva, Valencia) for the past 13 years and spoke minimal Spanish. She was headed towards the city to take advantage of the final sales at Marks and Spencer before it finally shut its doors to the public. She commented that she would especially miss shopping there for the lingerie because she had difficulty finding her (larger) size in the traditional Spanish shops.
I am sure Katykat would be an excellent resource as there are many enclaves of British and German expat communities throughout the Valencian/Alicante regions... especially in the smaller towns such as Denia, Oliva, Cullera, Peñíscola, etc.
Good luck with your idea!