Re: Oh no it wasn't.
This would be the service operated by the GPO that I referred to above. It was not cellular. It used a very limited number of dedicated channels, so it did not scale beyond a few police and rich people, and the network was frequently engaged.
The UK lagged other countries in developing cellular networks, Japan followed by parts of Scandinavia. It's no coincidence that Nokia had such a large presence early, as Finland was one of the pioneers. If Milton Keynes was in Finland or Norway, you might have made a cellular call in 1981.