In theory yes they can.
A European Debt Order (Possibly wrong name) can be placed on Experian and other Credit reference agencies lists which will affect your UK credit rating.
They upto the beginning of 2009 could not by law share credit info across borders, having said that the EU has ignored complaints from people who believed that Experian shared info between its offices based across the world.
Now I`m afraid due to movement of people between countries and not paying debts the law changed.
How far the company is prepared to chase you depends on a lot of factors, but if you are traced in say 8 yrs time, you could have accumulated a lot of interest and added extras.
You can read into here.
http://eu.eu/legislation_summaries/justice_freedom_security/judicial_cooperation_in_civil_matters/l16023_en.htm
A defendant’s address must be known with certainty for a European order for payment to be served. Service may also be effected on a defendant’s representative.
It can be expensive to issue, costs are then passed onto the person owing the debt, but you cannot get blood out of a stone and I have heard that anything under 3,000 Euros is unlikely to be chased.
So if your going to ignore it, don`t register on your electoral role, which will also affect your credit ability, don`t return to Spain, thanks to API the UK Govt now hold all passenger movements to and from the UK for a minimum of 10 yrs and share that info with other agencies.
So they could gain your address that way and then issue an order.
I doubt though for 800 Euros they would bother, but you never know, I guess a lot of Expats who have repo property will be most affected.