I watched the Gadget Show last night - watch it if a repeat is available as these were featured. In particular it showed that there is good connectivity in urban areas and no connectivity in some places.
They dowloaded a short video - it took 2 minutes by wifi in a hotspot, 7 minutes with a dongle with good connectivity.
It is impossible to recommend any one dongle - 3 has the cheapest 'unlimited access' but that is useless if there is no coverage where you want to use it.
I bought one last year and couldn't use it in Devon or Cornwall. So go to the websites and check the coverage before you do anything else.