Indian Ringnecks were very popular pets in the UK in the 80s and 90s. Over time, a number of them escaped from households, met up, and have gradually started colonising the British aisles.
They’ve bred for a few generations in the wild and can be seen flying solo or in large flocks around the country.
Look for a leg-band, an aluminium ring. If it isn’t wearing one, chances are its wild.
If it has a leg band, contact The Parrot Society in Hertfordshire who will try to get you to read the ring number :) this will have been registered when it was manufactured by ‘Avian ID’ in Cornwall. They in turn will contact the owner.
…but like I said, its probably wild.