What it boils down to is this: The CE standard is the new pan-european standard from compressed air bottles, replacing national standards (including the HSE codes in the UK) as of the beginning of July. New tanks for use in Europe should all be manufactured to the CE spec. HSE tanks will continue to be legal for use and restesting in the UK until the end of their service lifetime (usually 15 years). Whether the rest of Europe will continue to recognise either HSE or DOT standards, as they have done until now, remains to be established.
If you're buying a new tank, buy a CE type