HSE is the british standard for air ottles and DOT is the USA standard. HSE rated bottles exceed DOT testing and i think it is illegal to fill a DOT bottle in GB. Its also nigh impossible to get one re-tested. We covered this in an earlier thread only about 3 weeks ago...look back a few pages in techroom & you'll come across it. Basically, if youre in the UK, dont go for DOT!
It depends what you mean by pmi. If you mean a Piranha, then yes, they run fine on CO2. However, if it is a type with a Pure Energy inline regulator, get an anti-siphon tube installed in your tank -- regulators do not like liquid CO2.