Hi and welcome.
Thanks for the comments.
As regards the exchange visit, the one with info thats up on the site was arranged by the ex CEO of BT. I have a bit of a history with adsl going back to 2002 as an ex-adsl campaigner, so he was well aware of me and the site which probably had a lot to do with being given approval, which was for a couple of exchanges and with several 'specialists' in their own field.
I still regret not being able to visit the arranged BT tower visit though, several years ago due to work and other commitments. :/
If you are wanting to go round for interest or as a group, then I way I would approach it is by 'knocking on the door' of your local exchange and asking to speak to the exchange manager. Depending on the size of your exchange it may have to get approval/clearance by a BT area or regional manager. There is a lot of very expensive adsl equipment in the exchange.. and they also have to have security clearance for some equipment which doesnt even belong to BT but to the LLU operators (even I wasnt allowed to photograph non BT kit).
If you have a valid reason, or enthusiasm, or if its for an educational group visit, they may well oblige.
Good luck and let us know how it goes.