
it is 6 months from the date
the blood was taken that yielded the sufficient titre result.
That is for return to UK.
You do have to watch out for what rules other countries have for entry as for example Norway and Sweden the blood titre blood test must be taken not less than 5 months and not more than 364 days since a Rabies vaccination, so you couldn't travel through Europe to Norway with a dog with valid Pet Passport where the blood test was done only a short time after vaccination (usual protocol here 30 days).