I am looking for some statistics on online food sales over the last
2-3 years. I am based in the UK, so UK figures would be best. However,
US stats will also be welcome.

In particular, sales of Spanish food is of interest.

Dear John Duffy, 

I've found several pieces of information, in which you could find
interest. I am posting it as a comment because your question is about
to expire, and that leaves little time for further investigation.
However, whether you choose to re-post your question or not, here's
the information found so far.

Studies and market research on the question, US : 
2000 - "ARE INTERNET GROCERY MARKETS MORE EFFICIENT?" Michael R. Ward
http://www.ers.usda.gov/Briefing/FoodMarketStructures/conferencepapers/Ward.pdf
2001 - "eCommerce: B2C Report" Published by eMarketer - costs
$750-$850 - http://www.mindbranch.com/catalog/print_product_page.jsp?code=R203-034

Oct. 2001 - "Food-related business on the Internet" (Market Watch)
http://www.aphis.usda.gov/vs/ceah/cei/internetfood.pdf

+++
 
Legal and other implications for a business in the UK: 
"Distance Selling: sales of food and animal feed via the internet or
by mail order" / Response by the Institute of Food Research
http://www.ifr.bbsrc.ac.uk/science/consultations/distance_selling.html

I hope that helps, and wish you success in your intiative.