Frog, you asked about overpressuring tanks. It depends on several things. We are discussing a new, never used tank.
Since PST tanks are unavailable to you, we can dispense with discussion of those and that is just as well. However, Faber/Italy tanks are available in UK under different brand names including "Aqualung". I cannot, of course, recommend that you or anyone overpressurize a tank. However, I have observed that the Faber LP (180 bar) tank is sometimes overpressurized here in the US. The usual terminal pressure is about 200 bar.
Available literature in the US characterizes this tank as rated for 2400+ psi(DOT) meaning a terminal pressure of 2640. However, the literature goes on to say that the burst pressure of this tank is 6500psi; furthermore, the tank is capable of "10,000 cycles at 4000 psi", equivalent to 270 bar.
You can draw your own conclusions. I don't know the laws and customs in your area. In the US, it is unlawful for a "common carrier" (lorry) engaged in interstate commerce to transport an overpressurized tank. However,I've never heard of this law being enforced and it doesn't appear to apply to private use anyway. Beyond this, it makes sense to be prudent in the handling and use of high pressure cylinders. That means no abuse, no excess temperatures and an appropriate burst plug installed, if required. The standards and requirements for high pressure cylinders are understandably conservative and stringent as the originators of the codes have no control over the many factors which affect the cylinders.