Hard to answer within an NEC forum where the same rules will not apply. The answer is what ever that contract says is the answer!
From an NEC perspective there is the power to issue an instruction to do something that the Contractor is obliged to follow - and the wording (of clause 27.3) even uses the word obey. If this is something that the Contractor believes is not their risk and hence a compensation event, they notify accordingly and the PM decides if they agree and if yes asks for a quote.
I suspect the answer would be very similar under your contract. However - by all means the Contractor can TRY to see they will not proceed until they have agreed and see where it gets the,/what reaction they get and then decide what to do from there.
From an NEC perspective there is the power to issue an instruction to do something that the Contractor is obliged to follow - and the wording (of clause 27.3) even uses the word obey. If this is something that the Contractor believes is not their risk and hence a compensation event, they notify accordingly and the PM decides if they agree and if yes asks for a quote.
I suspect the answer would be very similar under your contract. However - by all means the Contractor can TRY to see they will not proceed until they have agreed and see where it gets the,/what reaction they get and then decide what to do from there.