Not sure how Brendan (webwatchmaker) feels about touching movement parts, probably needs to see the state first hand rather than photos and description, but it might be worth a bit of a chat.
Assuming it’s a case of no-go, and that Breguet know how to charge for the parts, then you’re left with trying to decide how much of a contribution to expect (full or partial, depending on whether you account for purchase price and age) or to simply reject it for a full refund.
On the basis of going with the latter, there’s been info posted here about getting refunds from HMRC, or you could probably find it easy enough searching online for it.
Did you compare the price with those on C24, within the EU, so as to save having to bother with VAT?
If the saving was more than the cost of a service, then you might want to hang on to it, and see what contribution you can negotiate with the seller.