This is a forum mostly geared to help with the devices themselves, not tell you what price plan to go with.
Why don't you check a few carriers sites and see what specials they have on? Not everyone on the forum is from the UK, we don't know their price plans or hardware costs off the top of our heads.
You're also looking for a 'free' device, but I'm not sure if you could get those devices for free where you are, they are quite expensive in Canada. Good luck on your search
If you want help differentiating between the devices and which would suit your needs best, then we're the people to ask.