This is vague, but this was shown possibly late 80's early 90's on UK TV and certainly a British production.
The movie (tv movie or possibly a 2,3-part series?) starts with the filming of a gardening programme, the TV crew are set up as well as the presenters to make a live broadcast. During the live broadcast one of the presenters unearths a body. The transmission is stopped and the police called in where many more bodies are found in the garden - all in perfect condition.
During the course of the movie it turns out that these bodies are actually sentient alien plants.
One of the humans ends up having a relationship with someone who turns out to be one of the aliens in human form and whose job it was to protect these other 'plants' and eventually find a way to leave earth.
At the end of the programme, the human who had the relationship finds the alien dead in the form of a dead plant in human shape...