Firstly, as far as I can tell, Virgin Mobile UK use stock UK BTU firmware. They don't have their own network branded firmware. Secondly, Virgin Mobile UK does not offer 4g, nor do they even have any plans to offer 4g at this moment in time. That's why you are only connecting at 3g.
Virgin Media might well use the EE network, who have 4g available, but Virgin Media don't offer access to that and your stuck with 3g connections.