I worked in IT for BT for 14 years - through the hayday of £15/share and all the Bolly you could eat, to where it is now, which is completely on its arse.
On one hand, you get to mess with a network on a scale you can't really imagine being in small business. Not only their's, but their customers too. You learn at a staggering rate of knots and gain huge experience. I worked all over the UK, travelled abroad and had a lot of good times. Private healthcare and a final salary pension weren't to be sniffed at either.
On the other hand, you are indeed a number, and when they slash and burn to offshore, you're just a wage bill and out.
Luckily I escaped, some of the lads I worked with are pretty much being either left to rot or bullied out of the company (because they can't do compulsory redundancy due to an old clause in the contracts). It's rotten to the core. Of course this happens elsewhere, as Deako alluded too - many places are stagnant, fine if you want to tick over for a decent wage, no good if like you you're looking for opportunity.
I now work for a big bank, I've been exposed to more in 18 months than I did in the previous 5 years at BT and I'm really getting on well. I had over £10k of training last year, and the culture is very much 'if you want to learn and progress, we'll push you on'.
My advice, if you're in IT, then there's pretty much all the contracting you can eat in London at the moment. Ignore what the Daily Mail says about the downturn, it isn't affecting London and IT. I've been tapped up a fair bit recently and a lot of others I know too by agencies offering jobs. Easy £500/day for bread and butter stuff. Get in with a decent employer and get experience. If you like the big bad world of Blue Chips, then hop on and ride it!