Well, 5.0 is the version for all European models at the moment.
That said, there have been 3 important security updates since November. Virgin have not released any of them. All other UK networks have, the last being applied last week.
If you know what you are doing it is possible to change the firmware without rooting the phone. I run an ex Vodafone S5 on Three with Three firmware which gives me access to their 4G Supervoice. (VOLTE)