Virtual networks sometimes have better offers than the actual networks themselves. Never having used one, I don't know if there are any downsides - coverage, uptime, time-limited offers etc.
It's always worth checking for deals on this site as things do crop up (I'm paying slightly less than you for 3GB, for instance, as a result of an offer last February).
If you don't want to switch networks, it might be worth considering a deal with another one if you have a dual-SIM device, although a simple upgrade may be cheaper (Vodafone currently have 5GB for £16 on a 12 month contract, for instance).
