The reason it can be done in some places and not others is simply the quality of the suppliers overhead cables - if it goes on singles, then the neutral must be the lower of the two cables, and there must be no joints in the combined neutral-earth wire, that rely on a single bolt, and there should be a minimum number of grounding pints in the installation to make it qualify as "PME".
Even so, there are a few lost neutral accidents per year.
Many older supplies were laid in during the postwar period and simply don't meet these criteria, and unless they are restrung, which is a process likely to take years, (there are plenty of fused neutral pre 1936 mains cut outs still in use for heaven's sake - these gusy dont move fast) and installtions connected to these can only be treated as TT.
There are also situations where PME is considered too dangerous, even if the supply integrity is not i doubt, usually where you odnt want to hold neutral and stand on true earth, - supplies building sites, caravans with a metal body, outdoor sockets generally and some kinds of farm machinary for example - so some sites are refused a PME connection on principle, as this sort of use is considered likely.
The other dangerous situation, where TT is prefferred, is when th ehig fault currents it causes can be dangerous, such as petrol stations, oil refineries etc.
Hope that helps.