Kilsby (2224m / 1 mile 666 yards) and Box (2939m / 1 mile 1454 yards) you can definitely see straight through. Those calculations might be slightly out as there are usually slight differences in exactly how long a tunnel is . Polhill Tunnel through the North Downs at 2370m / 1 mile 832 yards between Orpington and Sevenoaks can be seen through, as can Clayton Tunnel just north of Brighton at 2065m / 1 mile 499 yards.
Penge I believe you can stand on the platforms at Crystal Palace and Gipsy Hill and see through the tunnel to the next station.
I think the winner would have been the old Woodhead Tunnels closed in 1954. A book I've borrowed from the library, 'The Woodhead Route' by Anthony Dawson, asserts that when the first tunnel opened in 1845, the Sheffield Iris commented it was possible to see daylight straight through the bore from one end to the other. At 4840m / 3 miles 13 yards that's a fair distance. I think the second tunnel opened in 1852 was also straight but the new tunnel opened in 1954 was curved at the western end.
It seems impossible to find photos to prove this though, as with the narrow bore and huge numbers of heavy steam-hauled freight trains using the route every day, daylight would almost always have been blocked out by the sheer volume of smoke in the tunnel during its working life.
Chipping Sodbury seems to be the working winner named so far at 4065m / 2 miles 926 yards.