@alfie... that link mentions BT exchanges, but doesn't offer any places to download a POI set for them.
To be honest Nick, I'd be very surprised if a list of addresses for BT exchanges was publicly available, due to possible terrorist threats etc.
But maybe if you work for them or one of thair contractors, then you would be able to have access to a list of them? If so, we can suggest some tools that could create a POI list from them.
If you DO manage to find a ready-made a POI list for them, then installing to the TomTom is easy. You use the "MyTomTom" PC utility, click on it's icon in the system tray (next to the clock), select "Add Community Content" from the window that pops up, then click on "Add POI" and follow the instructions.