If a construction contract calls for fittings from a foreign county, is it legally permissible for a contractor to claim additional fees due to detrimental exchange rates?

There is no such provision within JCT Contracts. However, the contractor may be able to claim expenses under JCT Standard Building Contract 2009 clause 4.24 if a delay that is no fault of the contractor is caused and during this time there is a detrimental alteration in interest rates.