Neither you, nor friends and family staying at your Spanish property, need what is called a Letter of Invitation, as UK citizens have a visa waiver for visiting the Schengen travel zone - see the following extract from the Spanish Government linked below:
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"In compliance with EU Regulation 2018/1806 of the European Parliament and of the Council of 14 November 2018 British passport holders travelling to Schengen countries for holidays, business or to visit family do not require a visa for stays of up to 90 days in a 180 day-period, provided the visit is not for gainful activity."
https://www.exteriores.gob.es/Consulados/londres/en/Information-consular-services/Paginas/Visas/Visas-(FAQ).aspx
However, I believe the information on which press reports are based is the following extract from the Gov.UK website - note the use of the word "may" in the opening sentence:
"Additional documents required by tourists
At Spanish border control, you may need to:
show a return or onward ticketshow you have enough money for your stayshow proof of accommodation for your stay, for example, a hotel booking confirmation, proof of address if visiting your own property (e.g. second home), or an invitation from your host or proof of their address if staying with a third party, friends or family. The Spanish government has clarified that the “carta de invitation” is one of the options available to prove that you have accommodation if staying with friends or family."
https://www.gov.uk/foreign-travel-advice/spain/entry-requirementsThe Gov.UK information caused a lot of consternation at the beginning of the year, leading to The British Embassy in Madrid issuing a clarification - should a traveller draw the attention of border control security, the Spanish authorities could ask for any or all of the above information, but it's highly unlikely for regular tourists.
Personally, I've not heard of any British traveller being asked to provide this information.
Finally, you do not need an IDP to drive in Spain as a tourist, unless you still have the old style paper licence:
https://www.gov.uk/guidance/driving-in-the-euKind regards,
Kim