I rode in a few of these when I was a boy. there were a couple still in service in the town my grandparents lived in. They rarely saw more than ten years service as a London Hackney Carriage, the annual Public Carriage Office inspection was so severe mostly they were sold on to provincial town taxi drivers. And it seems quite a few made it to other parts of the world.
I think yours has a Perkins diesel engine?.
Tell us more about it before you move it on. Taxi cabs of that vintage are rare even in the UK!
Update, 2/13/21 the British licence plate (number plate to us brits

) shows it was first registered in central London, as probably most FX3's were. The licence plate normally stays with the car until it is scrapped or permanently exported. You can change it for a vanity plate as long as the new plate doesn't indicate a newer vehicle.
Roger.