The guidelines of the passport scheme are very specific, and providing that your pet dog/cat meet the requirements, then entry to the UK from Spain will be allowed without quarantine.
To travel and/or return to the UK, your dog/cat must have a valid EU pet passport, it must be microchipped, vaccinated against rabies at least 21 days before entry & treated against tapeworms.

Firstly your pet must be fitted with a microchip. This microchip is to ISO standards and the number can be read after being fitted.
Secondly your pet must be vaccinated against rabies. The vaccination must be carried out after the microchip has been fitted to ensure correct identification. We record this on your pet's vaccination record and pet passport the details of the microchip number, date of fitting and location, the date of vaccination and vaccine product information.
The final stage is parasite treatment - treatment against tapeworms must be carried out not less than 24 hours and not more than 120 hours before entering the UK. This must be carried out by the vet, who must fill in the appropriate sections of the pet passport, with the date and time, product name and information.

When traveling by air, there are certain regulations as to the size of the container that your pet can travel in. As a rough guide the container must be big enough for your pet to turn about whilst standing, to stand and sit erect and to lie in a natural position. Please check with your transport agency for full details of their requirements before traveling.

For information on pets other than cats and dogs please visit the DEFRA web site.
The information given on this web site is up to date at the time of issue but is not comprehensive and is meant as a guide only. For full up to date information on the PETS passport scheme please visit the DEFRA web site or Contact your Consulate.
For anymore information required, please see DEFRA web site www.defra.gov.uk or the direct link.