From 1 July (estimated date of enforcement), the Spanish government will require all arrivals to Spain from the UK to present a negative COVID-19 test prior to arrival (e.g. PCR, TMA, LAMP, NEAR) or proof of vaccination on entry.
Your NHS appointment card from vaccination centres is not designed to be used as proof of vaccination and should not be used to demonstrate your vaccine status.
All passengers entering Spain are still required to complete a pre-travel declaration form.
Your passport may be stamped on entry and exit unless you can demonstrate that you are resident of Spain and are returning to your place of residence.
Source: Foreign and Commonwealth Office.