Why is my range so poor?

I bought a Phantom a while back and it worked great. However, in the last few months I keep dropping control signal (video is still fine) and it performs an auto RTH. The problem has got worse recently - I can't go more than 30m without dropping out. There is a popup on the app saying 'Strong interference detected. Be careful when flying long distances', however I've flown it around the UK and everywhere has this issue - so its inside the drone or handset. How would I go about fixing this issue?