I Need To Fly My Shu Tzu To England She

i need to fly my shu Tzu to England she is almost 16 years ol d and has had a bout of bronchitis and found out that she has an enlargd heart consequently she is still coughing especially at night she is on medication her legs sometimes give way and she goes out to do her business but wants to go right back in in my heart of heart I feel her time has come but then she perks up she seems unable to settle down and is pacing a lot. I do not want to wait until she is in severe pain or choking. I have really been struggling with this decision. I feel her quality of life as she know it is slowly disappearing. she has made 2 visits to the vet = nothing is changing. vets never tell you when it is time I wish they would then maybe the owner would not have to struggle so much with that heartrenching decision. thank you j