This is so fascinating - did anyone do any more vent searching over the festive season - all you people that got digital cameras can use those to practice!
I worked at Dial House and it does have a fortress like quality to it. I also worked in a different BT building in Northants and I know that it was not only bomb proof but was indeed linked to the post office by an underground tunnel (albeit the post office was right next door). The building was 8 storeys high but was all disused apart from the 7th and 8th floors - a few engineers used a store room on the 4th floor and there was a big gym hall style room on the 3rd (why would BT need a gym?!?) but other than that, it was empty. When I worked there, it was for BT DFTS which do communications for the armed forces, which explains it's fortification, but not why we would need such a huge building for all of 100 staff max. There was also all the old operator equipment in the basement (presumably where the tunnel went from) which we were always threatened with in the event of any major incidents.
There was a vent I remember being at the side of the building, probably from the tunnel.
Slightly off topic there I suppose, but does prove that BT are shady and mysterious! Or something!!