This has been discussed in other forums but I'll comment- It has been my experience that Spain will not be the problem (unless you get unlucky). Watch out for the ticket agents in the country you are leaving from to re-enter Spain. They may require you to have a ticket with onward travel from Spain to your country of origin (citizenship). If the problem should arise, purchasing a fully refundable ticket from Spain to your country (assuming USA) would solve the problem. England is usually pretty strict so I would suggest you either stay clear of the UK or be prepared to purchase a ticket (having a credit card available is a must).