I recently took a dog (mini schnauzer) over to Belgium and was able to have it in the cabin :D , however only certain airlines will allow this, in my case BA wouldn't :( but Sabena airlines would :) . I flew out from Bristol and the staff there were very helpful. At first I had my dog in a traveller but the staff allowed him to walk around the departures

(as long as he didn't mess and was well behaved). The dog had to be booked on the flight in advance as apparently only a certain number of dogs are allowed on any one flight in the cabin, and the cost was equivalent to the excess baggage costs. On the plane I requested a seat at the rear so that disruption to other passagers was at a minimum and at this point he was in his carrier. Health papers are required and mine were checked on departure along with vaccination record however it might be that different papers are required for different countries. I contacted my vet, the airline and the airport so that I had all the relevant info before booking.