If it's new.....
If it's new chances are it will be stopped. Forget advice about listing it as a gift, customs have clamped down on this. Also under valueing won't work anymore either, as customs will now revalue items they feel do not show a true value on the shipping details. Undervalueing will also screw any insurance on the item, should it get damaged or lost.
Best thing to do is ship it with all the correct details and hope it doesn't get stopped. You will know if it has, as the item will not arrive on time and if tracked will show as being in the U.K. arrival depot for more than 2 days.
Warrenty return may be a bit suspect, unless it is shipped from a paintball buisness in the U.S.
If they suspect you are attempting to dodge duty, then you can be prosecuted as well!!