These reports have been collated by DEFRA (Department for Rural Affairs) and show escapes of non-native big cats in the UK. The original link on the DEFRA website can now not be accessed by this link Internet Archive will take you to the archived page. The page does take some time to download.
So there is governmental documentation to show that non-native big cats have indeed escaped from private collections, zoo's and possibly circuses. Specific places of escape have not been logged but I would suggest that individuals who own cats illegally would not have reported escapes to the authorities. It can therefore be suggested that these escapes cam from legal private collections, zoo's or circuses.
The list below is a direct copy of the DEFRA documentation but is not as detailed as the original documentation. For that I suggest you go to the aforementioned link.
Reports received by Defra of escapes of non-native cats in the U.K.
1975 to present day
Leopard Panthera pardus 01/01/1975 01/04/1975 05/01/1975 Recapture Kent
Clouded leopard Neofelis nebulosa 01/08/1975 01/01/1986 01/01/1976 Shot Kent
Puma Felis concolor 01/01/1980 30/10/1980 01/01/1980 Recapture Inverness
Caracal Felis caracal 01/01/1980 22/11/1993 Unknown Shot Kent (not observed)
Ocelot Felis pardalis 01/11/1981 01/11/1981 01/11/1981 Shot Unknown Lancashire
Jaguar Panthera onca 01/09/1982 20/09/1982 01/09/1982 Shot North Wales
Lion Panthera leo 05/01/1984 06/01/1984 05/01/1984 Shot Norfolk Norfolk
Tiger Panthera tigris 01/10/1984 19/10/1984 01/10/1984 Shot and Recapture Kent
Leopard cat Felis bengalensis 01/01/1987 06/03/1988 Unknown Shot Cumbria Scottish Borders
Bengal leopard cat Felis bengalensis 01/10/1987 06/03/1988 06/03/1988 Shot Cumbria Scottish Borders
1Clouded leopard Neofelis nebulosa 22/10/1987 30/10/1987 29/10/1987 Recapture Kent
Puma Felis concolor 01/01/1988 05/03/1988 01/01/1988 Accidents Leicestershire
Bengal leopard cat Felis bengalensis 01/03/1988 22/04/1988 01/03/1988 Shot Devon
2Leopard Panthera pardus 01/06/1988 21/06/1988 21/06/1988 Shot Kent
Jungle cat Felis chaus 29/07/1988 28/05/1989 28/05/1989 Road Casualty Hampshire
Jungle cat Felis chaus 03/02/1989 03/02/1989 03/02/1989 Found dead, cause? Cheshire Shropshire
Lynx Lynx sp. 01/01/1991 01/02/1992 01/01/1950 Unknown Norfolk
Lion Panthera leo 01/01/1991 09/03/1991 01/01/1991 Recapture Humberside
Snow leopard (Ounce) Panthera uncia 29/11/1994 30/11/1994 29/11/1994 Recapture Hertfordshire
Lynx Lynx sp. 05/11/1996 13/11/1996 28/11/1996 Recapture Oxfordshire Oxford
Asiatic Golden Cat Felis temminckii 01/09/1997 15/05/1998 Somerset. (not observed)
Bengal leopard cat Felis bengalensis 01/01/1994 01/01/1987 Shot Unknown Isle of Wight
Leopard cat Felis bengalensis Unknown 01/01/1994 01/01/1987 Shot Isle of Wight
Eurasian Lynx Lynx lynx Unknown 09/05/2001 08/05/2001 Recapture London
Leopard cat Felis bengalensis Unknown 22/04/1988 Unknown Shot Devon
Leopard cat Felis bengalensis Unknown 01/01/1990 Unknown Found dead, cause? Scottish Borders
Jungle cat Felis chaus Unknown 22/11/1993 Shropshire
1 Mistakenly simplified to ‘leopard’ in a Parliamentary Question response
2 Evidence now exists to suggest that this is not a credible sighting