An update for people like @sqw , I received my Pro Glowforge on 22nd, unfortunately it was damaged in shipping.
Customer service at GF was great and got a warranty replacement unit out quickly.
Again, unfortunatly the value on the customs form they sent with it was for the retail price, not preorder price or marked as a warranty replacement.
This meant the £690.49 Duty I had to pay for the broken GF (which I am claiming back via HMRC) was now $1388.90 for the replacement GF according to UPS.
GF again were quick to send me a commercial invoice for the correct preorder value and after a call to UPS this morning and eventually getting through to the correct department (ask the agent to be put through to Post Clearance Department) they asked me to email the corrected invoice to email@example.com with the tracking number in the subject field and will recalculate the duty for the preorder price.
Thanks to @sqw for the HMRC claim form over here… GF,UK Broken Machines, Warranty, Duty Refunds and stuff