Leezyjet, in the UK it is your legal right to be represented by a union. If the majority of the groundstaff workforce want one, VS is obliged to acknowledge this.
Although I am no longer VS crew, I can honestly say we had to fight tooth and nail to get a union in. And when we did, things did improve albeit only for the short term. I left Virgin for BA some time after.
I think what you have to accept here is the underlying culture around Virgin and bmi. They seem to want their crew to come join them, stay a year or so, then leave. I guess there must be some financial advantage there for them. Never having to give real financial incremements, not having to contribute much to pensions, not having loads of ladies on maternity leave etc.