On the NHS website

You can view and download your COVID-19 vaccination status online.
You need an NHS login to do this. You will be asked to create an NHS login if you do not have one already.
NHS login registrations may take longer than usual when there are high numbers of requests. You will need to register at least 2 weeks before your departure date.
Get your COVID-19 vaccination status: https://covid-status.service.nhsx.nhs.uk/