The start of Bruce's reply was frankly bewildering - I'd told him the amp was all packed ready to ship and he's now suggesting I pay someone to hack out the transformers?!?
The email did however conclude with an address for shipping the amp to. So I did just that - see part 27.
I wonder how much someone would charge to remove the transformers. That would save about 11 Kilos of weight.
Taking some digital pictures of the wiring inside the chassis to indicate where the wires were connected might be enough, or the wires could be cut and later spliced. We have transformers here and can use them to verify the work done, then your transformers would be re-installed over there. 200 Pounds is a lot of money.
Yes, you would mark that the product is Made in USA and being returned for repair. I would mark that duties were paid when it was shipped initially, and that it was only being returned from repair, not being sold.