All I can think is that the different customer service teams are being run almost in isolation, under totally different leadership styles and instructions.
As far as I'm aware, the Virgin Media call centres that we get put through to in India/Philippines are actually outsourced to Accenture. I'm in this business, so know that they will have tight Service Level Agreements (SLAs) to stick to, with financial penalties if they are breached. In most cases, the offshore/outsourced teams will be very limited as to what they can offer.
The way outsourcing contracts are done nowadays, they are more likely measured by how much discount/free upgrades they are able to avoid giving away (which gets reported to Virgin as Profit improvement) rather than by how many customers they have helped to retain or keep happy by handing out incentives. There is likely to be some Artificial Intelligence software running that provides the call centre agent with a "profile" of the customer they are speaking with, which will suggest what might be acceptable to them & rate their propensity to actually leave rather than just threaten to leave.
Just bear in mind that most of the offshore call centres are staffed by very low end resources, with high levels of attrition, and that they generally have a strict script to stick to, that they will be unable to move away from. It's not their fault, it's the way the contracts are written & enforced.
Some, if not all, of the UK based contact centres on the other hand are still Virgin staff & they have a much wider ability to offer things and go off-script. There will still be guiding principles that they have to work within & I'm sure that individual manager's styles will influence some of the variation there seems to be when talking with different teams.
Personally I always try to speak with either a Scottish or Welsh based representative when it comes to negotiating a new deal. I've been with Virgin/ntl since 2000 & have always managed to get a new deal that I'm happy with, without having to threaten to leave. I just "emphasise" what their competition are offering. I've just started with this years negotiations, so will let you know how I get on.
Currently I'm paying £89 for VIP bundle with 350MB BB, unlimited calls, Full House TV + Sky Sports & Movies, 2x V6 boxes... A Philippines based agent "offered" me the Ultimate Oomph package for £133 last night, which I "politely" declined.
I'm expecting a call back this morning from a different team.
My aim is to get the Ultimate Oomph package for as close to the £79 it's being offered to new clients for, although I'll accept if they keep it at £89. Also aiming for a 12 month contract with pricing fixed for 18 months, just so I don't have to go through the same rigmorole in a year's time.