I'd shoot for Thursday morning, too. Just to give yourself enough time incase your flight is delayed or you have to wait ages in line (it took me 2 hrs to check in because they spelled my name wrong on all the forms at the BA desk!)
Also, before you leave the vet's office, double check he wrote down BOTH the tick & tapeworm treatments! My vet did, however the vet of the woman next to me at Quarantine did not. She ended up having to wait 24 hours, slept in her car at Quarantine, and had to pay something like £90 for something that was already done! So, doublecheck before you leave.. just in case.
(And this was a USDA vet in Boston! Sheesh! My non-USDA vet did it correctly.

)