Hampshire and the south-east
Based on sightings in the NBN Gateway, there have been a handful of reports of water deer in Hampshire, the earliest of which appear to be near Basingstoke; one in Tadley in 1950 and the other two just south of Preston Candover, one during October 1953 and the second in 1972. There are also reports from Eversley in 1972 and near Andover in 1980, but the most recent sighting I'm aware of was just north of the A3, between Steep Marsh and Hawkley in east Hampshire, in April 2013.
In his Ph.D. thesis on Fallow deer (Dama dama) feeding ecology in the New Forest in March 1974, John Jackson mentions that occasional reports are received of sightings of water deer, but had not been substantiated. Similarly, the New Forest National Park Authority mention on their website that “Chinese water deer are only very occasionally seen in the New Forest”, although having followed up with them it seems there are no records and this is largely word of mouth. This appears to stem from rumours that the New Forest Wildlife Park near Ashurst used to keep them, although no formal records exist of this either, and the park hasn't had them since the current owner took over in 1997 and likely for longer. Andy Page at the Forestry Commission tells me he knows of no records of water deer living wild in the Forest, and I have received only two unconfirmed reports. A New Forest archaeologist and naturalist reported seeing one on Crown Lands in a small glade just north of Brockenhurst around 2000 - he was convinced it was an escaped pet. More recently, a local wildlife photographer described seeing what he was certain were water deer on two occasions, once in 2010/11 and again in 2012, both in a boggy area just south-west of Lyndhurst.
Further east, the NBN Gateway holds records for a sighting near Chichester in June 2016, one in Colgate near Horsham and near Bewbush in Crawley, both in 2000, a couple in Surrey's Mole Valley (Brockham in June 2018 and Newdigate in 2000), a 1972 report from Wokingham, and one from Farnham during November 1945.
Finally, there remains some ambiguity as to the presence of water deer in Kent. The latest revision of the Field Guide to British Deer, published in July 2023 by the British Deer Society, includes a map suggesting isolated presence in the county, although I suspect this reflects the 2016 BDS survey responses, which included reports on the north coast around the Oare Marshes, as well as on the western border of Kent roughly between Sevenoaks and Alresford. There is also a Daily Mail article, published in August 2020, that talks about water deer on the marshes. At the end of July 2023, neither the NBN Gateway nor iRecord hold records for this species in Kent, however. Furthermore, there is no mention of them in the Kent Mammal Atlas, which covers sightings logged between 2002 and 2012, and Stephen Hedley, the Kent Mammal Recorder, told me in September 2021 that he has no confirmed records of them in the county. Similarly, Kent Wildlife Trust Area Manager Steve Weeks told me:
“I have never seen this species on the reserve (or anywhere in Kent) and given that it is a very popular bird watching reserve, I would have thought we would have seen photos or reports if they were present.”
Both naturalists confirmed this still to be the case when I followed up in August 2023, and during my visit to Oare Marshes in 2021 I failed to find any obvious signs of deer activity (i.e., no slots, droppings, fur on fences, or obvious trails through the reeds). Some volunteers I spoke with at Oare told me that they'd never seen deer of any species on the reserve, although there were fallow deer relatively close by. The habitat nonetheless appears ideal for them, and there seems little doubt they could prosper here.
West and north
In the West Country, I know of no reports from Cornwall but, based on data held in the NBN Gateway and iRecord, one animal was observed alive on heathland on Dartmoor in August 2017, one near Colyton in south Devon during May 2018, and there are a couple of records from Somerset (2001 and 2008). More recently, in mid-August 2022, a sighting was logged from Swineham Point in the Wareham Channel (Dorset), although the abundance count was 10, which, assuming it wasn't a typo, seems very high given there are no other reports from the county. There's one report from south Oxfordshire (May 1996), one from Clarendon Park in Wiltshire (1990), one near Stroud in Gloucestershire (2007), one from an unimproved wetland field in Herefordshire (2023), and three from Shropshire, including two at Walcot Park (1950 and 1970) and one near Lawley (2020). In June 2020 a water deer ran across the road in front of Courtenay Williams' car in Warwickshire and there are two old records from Harrogate in North Yorkshire (May 1952 and in 1972), one from the Derbyshire Dales (2007), and only a single report from near Bettws-y-Crwyn in Wales, dating to July 1977.
Finally, Scotland appears largely water deer-free. Nonetheless, two records were recently submitted to The Mammal Society by Kevin Peace in Dumfries and Galloway in the south-west. Peace observed one individual in a field on the edge of Wigtown Bay in late June 2020, and two animals just north of Gatehouse of Fleet about a month later. Subsequently, Clive and Sheila Williamson submitted two reports from near Brunston Castle, Dailly, in South Ayrshire, during May 2023; presumably these were the same individual(s) on successive days, but there are no details in the records. To the best of my knowledge, these remain the only three reports of this species in Scotland. I know of no reports, anecdotal or otherwise, from Ireland.