I uploaded a PDF of the NHS app vaccination record the 1st time, when this was rejected I tried all different things. Rejected 4 times in total. Finally took a photo of the little card we were given and they accepted it! My IT skills are not at amateur level and I found it very frustrating to say the least.
I was at the stage of just taking the documents with me to check in as I know they will check them again. Anyway, all done now for the 14th August-just the dreaded covid test to go