iMac-delivery courier

I'd stick with the well-known ones who Apple use to ship their stuff. I've received new Apple computer deliveries from DHL and UPS and have arrived all in one piece.

Hopefully you still have the original box and all its packing materials which should help ensure a smooth passage.

Long time since I used Parcel Force when I was in the UK. I don't know what they are like now, but they weren't great then so I wouldn't want to suggest a company I'd had no recent experience with.