No courier is good; they all employ idiots who will throw your bike around and then pressurise the buyer to sign for it "in good condition" before they've had a chance to inspect it. Pack it as if it's going to be dropped from 1000 feet, remove derailleur and hanger and pad with masses of bubble wrap or old carpet underlay. Put a plastic transport packer in the front dropouts (shop will give you one with the bike box) and a spacer between the rear. Then get in touch with one of the many agents such as Parcel2go who will send TNT to collect it..... then pray.....
The cost will be about £25. I strongly recommend you pay the extra £11 or so for insurance.