doesn't "gassafe" only apply to tradesmen installing gas for profit?
while I may get slapped and deleted for saying this, it's possible to install your own gas and not be registered and all perfectly legal, however the install would very likely invalidate any kind of home insurance you have as in the event of a fire etc they'd just blame your DIY install..
having said that, unless you are a skilled gas installer doing your own home then it's STRONGLY advised that you let the professionals do any gas work for you as it's then their fault if your house burns down or blows up due to a faulty install..