What is the niche that you're aiming for as a VPS provider that isn't covered by the thousands of other providers out there currently? If you don't want to share publicly then feel free to message me but I'm wondering if perhaps you've not looked in the right places if you have something that you couldn't find from a provider already?
I wouldn't bother calling anybody for dedicated server pricing unless you need something really non-standard. As has been mentioned, the only wiggle room you have is if you're ordering more than a few where you might be able to sway some discount. It's pretty much a rat race and the pricing is within a few £ across all providers in the UK.
You can get live pricing on any providers website worth their salt. Any that say "call for quote" (as opposed to just giving you a base figure online) on something as simple as dedicated servers or colocation should be automatically removed from any list of contenders you're looking at as they just want to get you on the phone and start selling the world to you.
Rackspace pricing I can understand as there's a lot of factors that makes up the price but there are still a lot of providers out there that publicly list prices and features to give you a good idea of if they're having your pants down!