If you want to make a saving on hydro tests in the uk you can do so by checking in with HPAC at an event that they attend and collecting at a subsequent event, or asking your local dive shops.
The price can go down to a similar level to the US price depending on where you go, (and in the case of dive shops whether they want your custom - I paid once a good price for a fibre wrap then was later quoted more than double for aluminiums- either he didn't understand what size cylinder I was on about, or he did not want to do it)
A call to HPAC could confirm US tests are valid on UK cylinders, but as a 'bottle checker' I have an idea of how professionally or poorly some test markings are made on cylinders, but know how I can verify a testers ID in the UK
I don't know about verifying a US test result
An issue with taking your cylinder to the US is you need to remove the regulator to get it on a flight, which risks thread damage etc. (unless you have the right regulator which is compliant with air travel - old smart parts one I believe)
For some test locations in the uk you also need to pre remove the regulator, much easier for the user if the testers accept it with regulator on and they do it themselves with the right tools
