NASC yes it is access oriantated then there is numerous others like IIP Investors In People which proves you are a company that invests in its workforce and modern techniques and equipment, IIp is also a good course to claim grants for well worth looking into.
you also will notice management systems like ISO , OSHAS etc etc which in the right company doing high profile work are brilliant for helping open doors for big contract works government , local authority etc etc but if you a smaller company dont immediately go for them there is better ones more financially to go for first but do build towards these,
chas is a good tag to have on your paperwork it shows you hav been audited on your safety procedures among other things and you do the main stuff as it should be done, this also gets you a helping hand onto council, government contract work.
all can be time consuming , possibly expensive but good in long run , depending what work you are doing or what you want to do , doint just jump into it though check them all out and see which one fits your business best ok