I can,t see how a scope purchased abroad, cannot be serviced by the country you live in, I have moved around the world, and my leopold scopes are covered where ever I live. Contact Zeiss UK and insist they handle it. Don,t take the grey import crap, this is a smoke screen set up by UK dealers and was proved to be illegal when tried by the car sales industry.
I think you will find that depends upon the terms and conditions of the Warranty.
Some are restricted geographically - see, for example, the Zeiss Warranty in the US:
Customer Service ZEISS Sports Optics | ZEISS United States
"Only products imported by Carl Zeiss Optical, Inc. and purchased in the United States or Canada from authorized Carl Zeiss Optical, Inc. dealers carry this Limited Lifetime Transferable Warranty. Carl Zeiss Optical, Inc. will not provide warranty service for products purchased through unauthorized dealers."
And here's what they say on service and repairs of US goods:
"
Service & Repair Information for Products outside USA/CAN
Any product exported or sold outside the United States and Canada must be returned to the original point of purchase, with a copy of the invoice or your product registration information. CONQUEST Riflescopes purchased in Australia and New Zealand must be returned to their respective, local authorized ZEISS distributors.
DO NOT return exported items directly to ZEISS from outside the United States. DO NOT return exported items directly to Gentec International from outside Canada. ZEISS and Gentec International cannot accept products from, or ship products to, locations outside the United States and Canada."
Compare that to the Leupold Warranty:
"With the Leupold Gold Ring Full Lifetime Guarantee, if your Leupold product doesn’t perform as promised, we will repair or replace it for free, whether you are the original owner or not—forever (excludes electronic components). You don’t need proof of ownership. Or a warranty card. And there’s no time limit."
With regards to returning the item to the US, make sure before you do that you have a suitable Return Material Authorisation (RMA) from Carl Zeiss Optical Inc. This is important as it should prevent the scope being stopped by HMRC on return, and should also ensure that you don't have anything to pay.
With goods that are repaired free of charge under a guarantee or warranty, you can claim total relief from Import Duty and VAT. However you will need to produce proof of your guarantee or warranty.
But be aware that if you are charged for the repair then Duty and VAT may be payable on the charge for the repair plus certain other costs.
If the goods are replaced, VAT (though not import duty) is potentially payable on the full customs value of the replacement goods.
The manufacturer normally takes care of all of this, though it pays to be diligent.