EU legislation is split into 'hard' law (the regulations) and 'soft' law (the AMCs). Where only one AMC is published, that is the only means of complying with the associated Regulation. It is open to anyone to propose to their competent authority an alternative means of compliance with the Regulation but, until this is accepted, there remains only one means of compliance.
Take, for example, the requirement for a minimum of 100 hours of theoretical knowledge instruction for the PPL that exists in AMC1 FCL.210; FCL.215. Until the UK proposed to EASA an alternative means of compliance, all ATOs had to ensure that a minimum of 100 hours TK instruction was completed prior to taking the last of the PPL examinations. There is now no minimum amount of PPL TK instruction required in the UK but 100 hours remains the minimum in the rest of Europe.
In the case of FCL.050, there is no AltMoC in the UK and so the only way to comply with the Regulation is to comply with the AMC. You can propose an AltMoC to remove the requirement that all flights used to revalidate a SEP class rating are countersigned but until then it remains 'soft' law.