I use Parcelbroker.co.uk I think.
With their DHL bundles, they will take anything up to 30kg. My Marshall was heavy and cost about £11 to post and an extra £5 for insurance.
Oh, also has to be shorter than 120cm or 130 I think :(
A les paul, could be 4kg, + case + packaging isn't going to be above 15, and thats well within reasonable pricing. You can get a quote on the parcelbroker site for each delivery option (DHL, ParcelForce and TNT) very easily, have a go