LONDON — Confusion around two forthcoming European Union border-control programs is creating a fertile hunting ground for scammers, and consumer advocates say United Kingdom holidaymakers should double-check every online form before they hand over money or personal data.

LONDON — A ham-and-cheese baguette could become the most expensive part of your Spanish city break this October half-term. As British families head from London to Barcelona, Málaga and other sun-drenched spots, two different rule changes—one at EU borders, the other on arrival back in the United Kingdom—are set to reshape the airport experience.