Here's the information on entering Spain from the Spanish government ....www.spth.gob.es/info-pcr
It says: As of 1 February 2022, vaccination certificates issued by the competent authorities of the country of origin shall be accepted as valid 14 days after the date of administration of the last dose of the complete vaccination schedule.
I access my daughters Covid certificate from the NHS app using her email and log in (she's 14). The certificate with its QR code was available a day or so after the vaccination was done, so well before the 14 day limit. I always download a pdf certificate and print out a paper copy (because I'm old and analogue!). Maybe have a chat with your surgery if you're having problems logging in. Otherwise ask them if they can produce a paper copy. Just make sure it has the QR code.
If you're really nervous, a negative lateral flow covid test is fine for entry, although I think they need to be done within 24 hours of travel.
Make sure you fill in the Passenger Locator Form, you can fill one form for the whole family, giving details for each person. Look at the group/Family FCS form in the link.
www.spth.gob.es
If you can't upload any of the covid certificates, don't worry, just say so on the form and have it ready to show when requested.
The airline will check your forms at the gate before boarding, and will deny boarding if they're not correct. However it's a visual check normally, they don't scan the codes. On arrival in Spain (or Malaga at least) after passport control the QR codes on the locator form/s will be scanned, and any not-uploaded Covid certs will be scanned too. They also do a temp check with one of those infra red gun type things. Normally a very fast process, no hold ups.
In terms of anything changing soon, it depends on where you're going. Each area has the power to impose their own rules. We're still wearing masks, but some of those rules are set to change after Easter although not confirmed yet I believe. No news on requirements for certificates.
Have a fabulous holiday, and try not to let it stress you too much : )