OK - dealing with UK first.
To my knowledge [ and I have just checked with a mad cyclist friend ] you have to have reflectors [ red ] here.
Quoting mate ⇦all UK bikes - BY LAW - are sold with reflectors. He should have them. If not, they're cheap and worth picking up not only "just in case" but b/c they are a good idea.⇨
He also checked the CTC Site and from there got
⇦Spanish helmet law - 2005.03 Can you tell me whether it is compulsory to wear helmets in Spain? ANSWER It is - but with quite a lot of exceptions: like when it's hot, steeply uphill, or a built-up area! You may like to argue that it's always hot in Spain (compared to chilly Britain) but it's probably wiser to carry a helmet with you to put on when stopped by the cops. They have a trick of confiscating front wheels, to prevent further infringement by bolshy extranjeros! ⇨
Mate also provided this link for you
http://www.justlanded.com/english/spain/tools/articles/travel_leisure/general_information_and_useful_tip