If you get a boiler registration and ticket at your club it is good throughou
t Australia. This is not the UK where local tyrants can make life difficult.
You can approach Barry and Les on the AALS as I have done on another issue outside of the code and found them very helpful.
Having a CE marking means a lot. it means somebody has put their crown jewels on the line to warrant the boiler as fit for purpose.
On other UK boilers I am not so sure. But these days there are boroscopes which allow the build standard of the boiler to be inspected in detail. (mine cost about $49.00 at Jaycar)
UK boilers are still being built with girder stays on the fire box crown, these are no longer allowed by the AALS but there must be old boilers still registered in Australia with girder stays.
If all else fails agree to hydro test it to destruction, then you will know how good it is.