Assuming no deal Brexit we will have different licensing and MHRA will be responsible for approval new medicines and issuing MAs. The UK gov is setting about streamlining the UK approval process so it may be that we give approval for Covid vaccines ahead of the EU. EMA will no longer be in control of UK.
Pharma will launch first in markets that give earliest approvals. They will also have a commitment to pre-orders as and when approval is granted. UK is an attractive market as it is a reference point for many countries for approvals and price.
Also the manufacture and distribution will be phased, it's not as if they will drop 400m doses off at a warehouse in Germany.