Yes, all UK network providers offer unlimited MiFi deals, but be prepared to pay quite a lot for the privilege. One day, unlimited won't cost you any more than a limited deal – they'll all be unlimited, in fact – but we're not there yet.
It varies enormously from a few pounds a month to amounts that will almost certainly exceed what you pay for your entire home broadband connection. In some cases you will need to buy the MiFi device upfront, too, so you should think about that when making your choice and calculating how much it's going to cost you.
For what it does, sure. But the truth is most people don't need a dedicated MiFi device because they can get a broadband signal most of the time simply by being in range of wifi or from their mobile phone contract. MiFi serves a specific purpose, often as a backup when all else fails – you will know if you need it.
There are only two types of MiFi or mobile broadband device – one that distributes wifi to a number of devices and one that delivers internet to a single device, usually a laptop. Which is best will depend on your particular needs.
In some cases you will buy the device upfront. These can cost anything between £10 and £100, depending on its capabilities. If you are getting a mobile broadband contract, often the cost of the device can be folded into your monthly payment. On contract, these plans work similarly to your mobile phone plan – you will be tied in. Usually for 12 months.
Average speeds top out around 40Mbps, though thing may run quicker or slower depending on where you are as well as which network you are with. This really is a 'how long is a piece of string' question.