I just googled snakes in UK. Almost wish I had not, as I discovered this:-

The Aesculapian snake (now Zamenis longissimus, previously Elaphe longissima), is a species of nonvenomous snake native to Europe, a member of the Colubrinae subfamily of the family Colubridae. Growing up to 2 metres (6.6 ft) in total length (including tail), it counts among the largest European snakes

Escaped populations in Great Britain

There are two populations of Aesculapian snake which derive from escapes in Great Britain. The older one is in the vicinity of the Welsh Mountain Zoo near Conwy in North Wales This population has persisted and reproduced for at least the last 30 years.[9] A second, more recent population has been found in and around Regent's Park near Regent's Canal in London[10] and said to number up to 30. It is suspected this colony may have been there some years, undetected.