Do you have the packaged dimensions, because many companies charge either actual weight or dimensional weight depending on which is higher.To calculate dimensional weight multiple height by width by depth (in cms) and then divide that by 6000.
I don't think you will get much cheaper than the DHL quote for something that size. Most pallet couriers will charge you for lorry to pick up in US to take to airport, then charge for air/sea freight, then charge import duty and VAT (with extra for processing fee), then charge for lorry to more from UK airport to your house.
Companies like DHL/UPS/FedEx have drivers out in the local area already so won't charge for specific lorry to pickup or drop off.
Does the company you bought it from not have a courier they use? They may have reduced rates.