They are an MVMO and it's down to them to negotiate a deal with the 4G network of their host which they obviously havn't done yet. There's a very good arguement for not bothering with 4G yet, there are very few applications for throughput over 15mbit which you can easily get with HSPA so for the vast majority or people coupled with low 4G coverage, it's a gimmick anyway.
It's not to say that progress will happen, it's just that right now the demand for high speed is low.
Myshra has touched on the start of something here - for now I don't believe there's much that can beat a decent 3G network (5-6mbit say). However, there's a good argument for bothering with 4G if your alternative is a congested 3G network which struggles to deliver 0.5mbit then yes; an upgrade is good!
However, my attitude seems to align with that of Virgin; good 3G (one hopes)
So how are Virgin speeds (3G on EE) then ?