The tower in question is post WW2 and is built to MOD specification 5223A/51. It is one of several of that design still in use.
The integral metal ladder forms a siginifcant part of the structure and would not be easy to remove or convert. The best option is to add an external staircase (enclosed for weather-proofing) and new entrance door. The entrance door would require major surgery to the windows and supporting wall which would reduce the visibility from the control room. As that is a definite no-no from the CAA the only real solution is to knock the damn thing down and build a new one.
How much for a new control tower? Probably similar to Dizzee Rascals salary and overtime claims.
Incidentally there is an urban legend concerning this particular tower that in the RAF operational days a young officer rushed up the ladder to find his commanding officer at the top. He promtply stood to attention, saluted and fell backwards down the stairs.
