An international crew is required to comply with country of registration rules at all times. They are also required to comply with ICAO rules when in international airspace. They are also required to comply with the rules for the territory or state over which they are flying.
How does the average crew do this? From my experience, particularly with Asian airlines, is that they don't (many do not cover this in training nor do they provide copies of relevant regulations (such as in the Jep) for the crew to refer to enroute).
Does this ignorance provide protection for the crew? No it doesn't. How many of you are aware that you will be subject to jail and/or seriously big fines for offences relating to paperwork and skill errors in many countries in Asia particularly? Do you think the argument "hey I did not know that!" will cut you any slack?
When flying to or over other countries, such as the US, Australia, NZ and such, the permission to do so for your airline will have been negotiated and approved, and one of the documents that you, as crew, need to know about and read is the Operations Specifications(s). It should cover the major requirements of routes, aids, airplane registrations and especially fuel requirements. when you have an Ops Spec it overrides the regulations as printed in that country's regs, and might very well be more restricting.
It is up to the carrier to advise their crew of the requirements of the Ops Specs, but as I said, many carriers fail to do that. Does that protect you? ("hey, I wasn't told!"). No, it does not.
If the BA crew was not in conflict with the CAA regs, ICAO, the FARs or their own Ops Specs, they are fireproof. If they had not done as they did, then they might have had to answer to the airline for why they did not, and justify the costs and inconvenience. It is an engine this time, but the same problem could be caused by any mechanical defect and so long as the decision is backed up by procedures, experience, common sense and consultation (CRM) when applicable, why lose any sleep over it? The captain is paid the big bucks for this very reason.
You don't like it? Get the rules changed.