From the Spanish Tourist Office:
Do not do what many people suggest and ignore it.
There is a
legal convention in place (and there has been since 1964) allowing European countries to seek enforcement of fines for driving offences in the home country. There was a
further convention in 2011, allowing cross-border sharing of information on road safety related traffic offences, which includes speeding.
I'd have to doublecheck whether the 2011 convention has come into force in the UK yet - but even if it hasn't, it can be applied retroactively!
EDIT: Just checked. Part 6 of the Criminal Justice & Immigration Act 2008 implemented COPEN 24, in relation to the enforcement of fines for speeding.