Sorry but its the usual situation - If you want the work you need to comply with the clients wishes noting that you should put your case forward in a strong fashion as usually, but not always, 18001 undertaken properly means something more than many such schemes

As Bob says, you've usually no real option but to jump through the hoops - unless of course the client is a LA, Government body, Health Board etc. Procurement & competition Law prevents these sort of bodies from making such stipulation, and you can challenge that. Sad to say that some such Organisations either do not know, or choose to ignore this! In a more commercial context, I am personally supportive of the efforts of SSIP in attempting to bring these PQQ bodies under one umbrella.