Hello Natalie, I'm Dr. Deb.
I recently came online and see that your question about Poppy hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
While it's not likely that Poppy would develop hairball issues at her age (especially since she's never had such problems in the past), it's possible, I suppose, that this could explain the behavior you're seeing. Sometimes a large wad of hair can become lodged in the stomach and essentially act as a foreign body which triggers attempts to bring it up.
If this is the case, then any over the counter hairball laxative designed for this purpose should be helpful. I'm certain that the store located near to your home will have just the thing. I usually suggest 1-2 inches, 2-3 times a day. These products typically are quite safe but will occasionally cause softer stools if too much is given.
Butter is not a very effective lubricant for these cats so I'm not terribly surprised if it hasn't worked.
It's also possible that she has esophagitis for some reason. If so, then over the counter, human Famotidine (Pepcid AC) may be useful. The dose would be 1/4th of a 10 mg tablet twice a day. This drug may be difficult to find in the UK so an acceptable alternative would be Omeprazole at a dose of 1/2 of a 10 mg tablet once a day.
Sometimes it's difficult to tell the difference between attempts to vomit and coughing/gagging associated with the respiratory system.
If this latter possibility exists, then there are other possible explanations for her behavior.
1. Feline Asthma which is similar to this condition in a human in that there is inflammation in the lungs which is triggered by something in the environment such as pollens, smoke, aerosol sprays, etc. Poppy is about the right age when this condition can develop.
This condition can often be a difficult diagnosis to make. Chest x-rays might be suggestive as would bloodwork (eosinophils might be elevated but they aren't in every case). I often diagnose this condition based on a cat's response to steroids. If they are 100% better, then this is the diagnosis. It may seem like a crude way to diagnose this condition, but if I rule out other possible reasons for the signs first (see below)then I don't worry about side effects from steroids.
This LINK discusses this condition in detail.
2. Bordetella or mycoplasma infections can cause respiratory issues which can often be confused with gastrointestinal ones (attempts to vomit).
Doxycycline 5 mg/kg twice a day can be given to rule out these diseases. This drug has the added benefit of anti-inflammatory properties
3 . Lung worm which is a parasite that is most often diagnosed with a fecal sample although these cats are coughing more than they are gagging or hacking.
This condition would be less likely in an inside cat.One effective treatment would be panacur.
4. Heartworm disease. This disease is transmitted by mosquitoes and depending on where you live in the UK, may be a real possibility. Again, most cats are coughing, though.
We do have a test for this condition which is often done in a vet's office but it's reliability has recently come into question. Unfortunately, we don't have an effective treatment for this disease in cats. This disease is different in a cat than in a dog and the symptoms are related to inflammation in the lungs. As a result, steroids can often be helpful.
5. Upper respiratory infections are at the bottom of this list for a reason. These cats are usually sneezing and have discharge from the eyes and/or nose. Stress can trigger an episode if a chronic condition and acute ones can be seen if the cats go outside.
I hope this helps; again, my apologies for the delayed response to you. Deb