Price is based on what market will bear - whatever that market is.
Nothing to do with cost or value or even exchange rate (the latter of course affects the manufacturers profit - as rates fluctcuate they can potentially do better or worse)
There is just no way they could apply a straight exchange rate to UK prices and have them sell
any units in the US against competitive entities.
Spares are the same thing - and not just Triumph! I just priced out a list of Suzuki parts for a UK friend and they were a smaller number in dollars than the UK in pounds i.e. less than HALF! The actual numbers were $785 vs 850 pounds.

Even accounting for Duty, VAT & shipping, savings would still be about 700 dollars.