ParcelForce seems to be the one. Most every lamp I purchased from the UK was sent through PF. Never had any damages or losses. Your best bet is to get a quote from them...be vary accurate with dimensions and weight. Its not cheap.
Parcel force / royal mail is fine I would just use the royal mail postage calculator (http://www.postal-rates.co.uk/) to work out how much it is going to cost you before you get to the post office.
The main thing is package it well - ideally in the original packaging if you still have it and make sure you include the weight of the lamp as well as the packaging.