Been quoted £45 for a full test, I had two quotes, one from a good gun shop the other from a dive shop.
Both stated the cylinder is sent away to be tested & will take a few weeks.
It seems then you don't walk into a gun shop with an out of test cylinder, wait 5 min's & walk out with a test done.
The nearest test center to me apparently is Birmingham.
If you find info with the search option you'll find the test is quite lengthy & worth the money, if a cylinder goes pop It's a rather loud & dangerous pop.
Store away from damp areas, rust is not a good idea.