On two occasions I have seen OcUK staff on these forums proffering discounts. The first time was a guy looking to buy £6K worth of systems in one hit, and the other was in the ASUS MARS thread where it was said that a discount could be organised for someone who wanted two GTX295 MARS cards.
OcUK also do 10% off 'vouchers' for business buyers and for people who haven't shopped with them in a while (a year I think).
So, yes, they do.
It's also worth bearing in mind that a credit card transaction costs OcUK something like 2-3% of the total value of the order whereas a Maestro transaction is a fixed fee (about 80p I think).
So on a £1000 order you could ask for a £25 discount if you paid by Maestro vs. a credit card.