Virgin is pretty reliable. They used to use the T-Mobile network. They now use the EE network (Which is Orange & T-Mobile) AFAIK, they are the biggest network in the UK at the time being
Because it's so large, it takes a bit longer to improve the infrastructure. I know how long things to take to even move things from one place to another in large organisations (It takes us around 6-8 months just to move some servers to another place in the same damn room, because of bureaucracy and risk)
To roll out 4G would take a bit longer. That being said, they'll definitely do it probably relatively quickly, because the business case for 4G is compelling ie: the risk is worth it
And with VM's EE partnership, Virgin media business can perhaps also sell some of it's services back to EE, to help move us forwards with 4G. It's a nice little domino effect
Most networks offer 3.5G atm (or HSDPA)
Are EE any good in your area? Also, it depends on the phone you have. I think my phone (Nexus 4) doesn't support 4G/LTE, but the nexus 5 does
Older versions of iPhones don't support 4G/LTE either
PErsonally, I find that 1GB is plenty for me
I tend to use just under 3 hours of minutes each month (So around 180 minutes), barely any texts (cos WhatsApp), and around 500 - 900Mb/data per month
As I said before, Virgin are bloody good for retention deals as well, especially if you're a Virgin media customer?
tl:dr - If you don't need the "unlimited" data go with Virgin. In around 3 months time, tell Virgin that three gives me x y and z, for 10p less AND it's 4g. You'll probably get a discount ie: you'd probably pay aound 20-25% less than before
If you have an android phone (Might also be an app for other phones), download "call metre 3G" or is it "call meter 3G" Any way, download that, to see exactly how many minutes you are using per month
Currently I get
Unlimited minutes
Unlimited texts
Unlimited VM to VM
Unlimited calls to landlines
Unlimited internet
Nokia C3-01 (that I sold)
For £17.49/month
As I don't need Unlimited minutes, texts, VM to VM, landlines, or unlimited internet, I'm happy to go for the £10/month sim only rolling contract