OK, this one is hard to find on a fast search, but...
Per the British Tourism website... import restrictions:
Offensive weapons, such as flick knives, butterfly knives, push daggers, belt-buckle knives, death stars, swordsticks, knives disguised as everyday objects, knuckledusters, blowpipes, spring-operated telescopic truncheons, and some martial arts equipment.So this would imply that a regular knife could be imported.
It is legal in the United States to mail a knife.
No DHL has a page about import restrictions:
http://www.dhl.co.uk/publish/gb/en/information/country_details1/import_guidelines.high.htmlI do not see any weapons restrictions, other than what would obviously be restricted by law.
From what I could tell, if you could find a dealer in the US to ship to England, you would only need to find a way to secure that you are 18 years of age or older so that the shipper would not be held liable for charges stemming from the Offensive Weapons Act of 1996.
I did note that this site
http://www.bkcg.co.uk/ does ship knives to the US without restriction (but our knife laws are a bit of a joke) so there does appear to be some avenue for shipping knives.
I don't know if England has a Curios and Relics Law regarding certain types of knives, but it might allow for shipment in cases were a normal knife would be restricted.
If you were here in the USA, I'd just take you to a gun show, someone always has a couple tables of knives tucked in between the rifles and the shotguns.

-Lego