Hello Tanya.
The Requirements
To bring your pet INTO the UK under the Pets Passport Scheme, follow this sequence to the letter -
Have your pet micro-chipped so that it can be identified.
Then have it injected against rabies by a qualified vet after it is micro-chipped ( this has to be done even if it has a current rabies vaccination before it is micro-chipped ).
A blood sample from the pet must then be tested by a qualified vet. to prove it has an acceptable level of protection against rabies.
Assuming a successful outcome to the test, the vet. can then issue the Pet Passport
Then, THE MOST IMPORTANT BIT. Your pet cannot be brought back IN to the UK for six calendar months after the date of the original blood test. So if you begin the sequence tomorrow, you would be able to take the pet OUT of the UK as soon as the Pet Passport is issued but you couldn't bring it back IN much before the beginning of September.
'Val A' has very adequately covered the timing of the treatment required prior to return which gives you two options. If you are going to use a vet. near to Guines, book your return trip after about 11.00am because the pet will have to be examined before you check-in for the ferry / train ( at least an hour earlier ie. 10.00am ) and as it must have been treated by the vet at least 24 hours before ( and the vet. is unlikely to accept an appointment before 9.00am on the previous day ), that's about the best you can do. For the same reason, a return journey on a Monday can be problematic unless you can see the vet. late on Saturday morning.
If you prefer an earlier channel crossing, there is a second option. Arrange the treatment in the late afternoon at a convenient town on you return route ( within say 250 miles of Calais ). Then, by the time you reach Guines, 24 hours will have elapsed so there will then be a further 24 hours to catch the ferry or train.
Hope this helps,
Olly.