I may have some experience
What position you going for?
Like most big corporates, its usually criteria led interviews. ie - "can you give me an example of where you've managed conflict?" Then they probe down until they get the answers they need. All fairly standard HR stuff - there is probably a fair bit on the web about it.
Am in the process of leaving BT - but I can say that its a reasonable place to be - they have a deserved reputation for looking after their people. Having said that, it can be extremely bureaucratic, slow moving and frustrating.
The real question is - which bit are you going into? BT is massive, and some bits are much better than others. (and GP3 - my own personal experience of working in the bit of BT you've linked to is not one of "high standards - professionally and technically"!!)
It has pros and cons just like everywhere else, to be honest. PM me if you want any more details.
BT Retail - some field engineers who dig holes in roads. Lots and lots of call centre people selling stuff/fielding complaints. Plus all the associated management gubbins
BT Wholesale - some field engineers who dig holes in roads (again). Plus lots and lots of management gubbins relating to keeping the network running. (planners, schedulers etc)
BT Global Services - the IT bit, does networks, big IT projects in public and private sector.
Its got about 100000 people, so there should be something in there for you!