Whilst I realise that's it's a legal requirement to apply for residencia for a stay exceeding 90 days, I'd just point out that - until the end of this year - the length of stay of UK citizens isn't monitored, so I don't see how the 90 day residencia rule can be enforced.
Next year will obviously be different, and assuming Spain apply the Schengen procedure, anyone staying more than 90 days will be told to leave Spain but will be given a period of grace in which to do so. Under the Schengen rules, it is possible to apply for an extension to the 90 days, but it will be necessary to provide a convincing reason for such an extension.