What are the restrictions for food from Spain that can be brought back to London?

Answer 1

If you bring food or plants back to the UK for your use this is called a ‘personal import’. Personal imports of meat or meat products and milk/dairy products are banned from most countries outside the EU.

There are also strict controls on bringing the following products into the UK:
potatoes
plants
fish
shellfish
honey
eggs and egg products
certain fruit and vegetables
The same rules apply if you are bringing these food items into the UK yourself, if a friend or a relative is bringing them as a gift, or if they are sent in the post.

If you bring back banned items, or items in an amount that exceeds certain weight limits, all of the goods will be seized and destroyed. If you declare these goods at the HM Revenue and Customs (HMRC) red channel (‘channels’ are areas you walk through to leave airports, international train stations and ports) you will face no further action (for example a fine or prosecution).

If you go through the green channel and HMRC Officers find undeclared items you could face long delays, in addition to the goods being seized and destroyed and you may be liable to prosecution.

I think for more information you can just click: http://www.defra.gov.uk/animalh/illegali/allow/allowances.htm

Answer 2

No meats, no milk and milk products like cheese and no animal products that have not been treated (like feathers, raw wool and so on.)
If these rules are not applied right now, they can be used any time a disease is found, without previous warning.

At this moment it seems that the rules within the EU are not in use and you can bring almost anything, but check with the websites below.

Best check the website of the British customs the day before you travel:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageTravel_FAQs&propertyType=document&columns=1&id=HMCE_PROD_009841
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageTravel_ShowContent&id=HMCE_PROD_010221&propertyType=document