Wigan is actually a good nomination for this thread. The constituency wasn't created in 1885 as David suggested; it is actually (surprisingly to me) an ancient borough going right back to the Model Parliament of 1295. And it is one of the relatively few boroughs that were 'waved through', so to speak, by the 1832 reform without any boundary changes at all. The 1868 boundary review proposed the addition of Ince but this was rejected so the ancient boundary stayed in place and was again left unchanged, so far as I can see, in 1885 (although it was at this time that Wigan lost its second seat). The pre-1832 boundary thus survived unchanged until 1918, when Pemberton was added to the seat.
Such longevity of a pre-1832 boundary is not unique, but it's pretty unusual.
I think most people would be surprised to learn that Wigan is the only ancient borough in Greater Manchester. (The reasons for Manchester's lack of borough status seem rather obscure.)