I agree with
@HSTEd that the EU will block exports. Its in a desperate situation and its not just exports to the UK, its Canada and other countries.
The UK appears to have sufficient stock to get 20 million people both doses on schedule but most of the Astrazenica needs its filler and to be bottled (in Wrexham). Assuming an EU export ban then Pfizer stocks would be mostly used for second doses and Moderna wouldn't arrive to either the EU or US lifted export bans. US rollout should finish during the summer. Without any of new vaccines there will inevitably be a slow down in giving of first doses in second quarter.
Back of fag packet calculations:
UK production of Astrazenica is 2 million doses per week (finished product). 7 million doses of Pfizer have used, imported or should be imported while EU takes a week or so to introduce an export ban. That is a total of 59 million doses available in first half of this year in event of EU export ban. That should be 2 doses for the 20 million high risk people (approximately one in first quarter and one in second quarter) and 19 million first doses. Third quarter Astrazenica doses would need to be mostly dedicated to people who got their first dose in second quarter. UK made Vaneva and Novavax should be used from July, along with J&J (only requires one dose) and Moderna imports. That means the remaining 28 million people should also be fully vaccinated by end of third quarter (but with shorter interval between doses). Production and imports in the third quarter should significantly exceed the first two quarters combined.