In terms of airport checks, at Heathrow on Sunday there were none at all.
I was expecting my passport to be flagged at the automatic gates in T5, as I hadn't managed to complete the Passenger Locator Form with the evidence of my two jabs. When I tried to upload the QR code for my second jab the form wouldn't take it, saying that it was a first jab QR. (Which it wasn't)
So I completed the form by ticking the declaration that I was double-jabbed, and that I understood that I would have to prove this on arrival.
In Spain they visually checked the PLF and my vaccine pass, but nothing was scanned. Departure was complicated further by the fact that the Iberia check-in staff could not believe that a 16 year old DS with me didn't need a PCR test through not having a double jab. We were almost denied boarding because of this - and it just about gave me heart failure for the 20 minutes while she looked for evidence that I was correct. (Stoopid UK rules)
So I sailed through LHR without anyone in the UK checking that my declaration that I have been double jabbed was correct, even though I should have been stopped and my documentation checked. Its the worst of both worlds: costly but totally ineffectual. Contrast that with arriving in Spain - both QR codes scanned, evidence of DS's PCR physically checked, and random temperature checks. Reassuring: I don't mind jumping through hoops if there is a clear purpose to them and my time isn't wasted.
Anyone entering the UK at the end of half term can probably bring back as much covid as they like - although it's a lot harder to find any in Spain.