Not strictly from the UK, but it was at the time of building in 1897:
MGWR 84, Clifden, Ireland
From my rather dull and vague notes on it:
MGWR 84 was built in 1897 as a 5-compartment third in Broadstone, according to Atock’s standard design for 3rd class modernisation. Seats 60. Two endmost compartments had a half-partition (half-back) between them, and a ladies-only compartment was present at the other end. One of 77 built between 1890 and 1900. “M,” suffix added to Midland stock after GSR amalgamation. Became a second in 1954 when third class was abolished. Became departamental 467A after withdrawal (1964?).
Preserved by GSRPS in Mallow in early-1980s, and saved by private members when the group collapsed. Arrived at Clifden Station House Hotel in 2000-and-something for eventual restoration for their museum (ex-trainshed). Restoration (which probably won't happen) will probably cost about €75,000.
It's not really much of a shame, I suppose, we already have two others the same and a third which is very similar.
There was the very tattered remains of a GNR (of Ireland, that is) 6-wheeler near my house that I was eager to see, but unfortunately just before I was going to get a proper visit to it last year I had learned that it had fallen apart
http://s0.geograph.org.uk/photos/34/65/346570_80f19ddf.jpg
As bad as it looks, it would have been interesting to see. However, I think the one at Downpatrick and the one in Omeath are similar enough