The origin of the UKs priority access to the AZ vaccine was due to their early funding of the Oxford vaccine on condition of 1st priority. This was before AZ even got involved. In fact the UK govt had such deep involvement that they were able to veto a deal between Oxford and Merck to manufacture and distribute the vaccine over fears that it would allow Trump to block their priority access through export controls (as Merck would manufacture in the US) so the UK make Oxford partner with AZ. AZ inherited that pre-existing deal between Oxford and the UK govt.