As a consequence of Brits now being a Non-Eu citizen, border security within the EU has tightened which includes more bureaucracy for British Citizens. In Spain the following rules may require you to produce evidence of your stay as follows:

1 Passport (obviously)

2 tickets for travel both to and from Spain.

3 Evidence of Travel Insurance

4 Evidence of sufficient funds for the period of your stay in Spain

5 Evidence of your location address in Spain (Hotel or rented apartment/ place where you are staying or ‘Carta de invitacion’ completed by your proposed hosts)

These requirements mainly relate to tourists, visiting family or friends or attending business/cultural/learning or sporting events insisting other countries within the Schengen/EU area then ensure your total time falls within the 90 days. If you overstay your welcome, you could risk being banned from entering any Schengen/EU country for up to three years.

This is why there is now a requirement to have your passport stamped on entry and exit. British nationals residing in Spain don’t require their passports to be stamped

In reality some Border checks may be less strict than others but you should be well prepared if you are selected at random to provide such documentation.