Hello Bazz.
Your installed system has been designed, manufactured and approved for working with the new 30mB standard, as have the fitted appliances.
Under Health and Safety Law, it is an offence to use or install a system where the appliances are not supplied with fuel at the rated pressure, and thus the older 28mB Butane and 37mB Propane are legally incompatible.
This does seem rather ridiculous, as the 28mB butane is very close to the new 30mB and will probably fall within the accepted working pressure tolerances, but the law is the law.
To comply, you would need to have the system including all the appliances re rated and badged for the pressures you want to use, and a certificate of conformity issued by a certified fitter.
To do the conversion without having the system re-certified could render your insurance invalid, and leave you open to criminal prosecution under HSE regulations.
( All HSE prosecutions come under criminal rather than civil law)