The short answer is they don't and it's pointless to ask about it since these flights are conducted under part 91 for private aviation. There is almost no charter or scheduled flights allowed on single engine aircraft in Europe and the few that do exist are using single engine turbines on an exemption (the exemption is based on the fact turbines have a demonstrated order of magnitude better reliability).
This has changed relatively recently (2017) in the EASA rules and single-engine IFR charter flying is possible without special exemptions as long as the engine has demonstrated good reliability (basically it means turbine engines).