It's very possible. If the snake in question is a CITES listed animal, then CITES export documents will of course need to accompany the animal. Be sure to include a copy of the CITES paperwork both inside the container and taped onto the exterior. I've heard accounts where the sender only included the documents inside and customs officials refused to open the container and returned package to sender.
CITES documents can take anywhere from 3-12 weeks (usually around the 4-6 week range). If completed correctly, there are typically no problems acquiring these. Most delays/refusals are due to incorrect/incomplete application submissions.
As for selecting an airline, not all airlines will ship live reptiles (too many bogus insurance claims in our hobby). I know Air Canada accepts live reptiles and they do service the UK. You can call their cargo office for more details.