Pros
Work-life balance and time off is incredible. Nearly 35 days PTO a year and you can work from home as often as needed. Office culture is great, and everyone is very amicable. Friendships are forged through the commonality of the overwhelming frustration of getting anything done.
Cons
Senior leadership is terrible and any time there's a change, it's just a shuffle of the same people, even when the CEO is inevitably let go after 3-4 years because success has been meager the past decade. Very rarely are outsiders considered. The MD of Commercial, for example, is an outright mean and in most respects terrible person, and not a single person understands why he's allowed to have his job other than he's been working as an exec already too long. Lack of vision. No one knows what the company should be good at, so BT is only barely competent at most things it does. Death by commitee. There are typically a dozen people who need to sign off on any given action. If even one says no, it all falls apart. Internal systems. Most haven't been upgraded since 1998, and while there's a whole push to transform them, it's being hindered by the above. One person has said no and it's all beginning to get delayed. British bureaucracy. BT was a state monopoly in the 1980s and many in management roles are still around from those days. Some are great, others have engineered themselves so tightly into their jobs, you'll need the jaws of life to get them out. Any type of change causes pure panic, but the British are too polite to voice their negative opinions of terrible ideas. Talent crunch. Most of the best talent moves on rather quickly for better pastures, and the bottom tier are fired for cost savings. You're left with a glut of mediocre middle-managers who would probably be analysts are most other major companies. Pay. Good luck ever getting a raise even if you're promoted. Anything over 10% is typically rejected or spread out over several pay reviews, and if it happens mid-year you'll have to wait until June for it to take effect. It directly contributes to the talent flee mentioned above. Career growth. Mangers will always find new and exciting projects for you, or even new teams and titles, but at the end of the day, until the man/woman above you retires, you're going to be stuck at your level unless you do some serious politicking.