The first two shots show that its pretty obviously a disused canal tunnel and I can tell you that the top one shows the Eastern portal. The second and third photos show that its in pretty good condition, which is perhaps not surprising as it was built towards the end of the era of canal expansion.
On the original of the one above you can just make out a spot of light in the far distance which gives you an idea of the length. The only other clues I can give you are that the whole of the canal concerned is disused and has been for some time and that its in the southern half of England.
I wonder if anyone knows where it is. (If not, I'll tell you later in the week!)