I assume your property in the UK is worth more than €50,000, correct? In which case yes, you need to declare it on the 720 tax form, and you could be facing a fine for late declaration. No need to panic, but if I were you I would seek specialist tax advice without delay.
You can find out more about this topic here:
In particular this article: Expats are the target of a new law on worldwide asset declaration
An interesting news report on the subject (in Spanish)