as you have found out, £20 bag is NOT unlimited everything
it is "always on"
and as you have discovered you only get 6gb as usable data, after 6gb data speed is restricted to 256kbps, (which is less than 3g) between 8am - midnight, also it is not really set up as a replacement for broadband internet, after all it is only a PAYG mobile phone service?
gg are upfront, although they don't make is very easy to read the info, but it's there
good luck with your new service