Yes, all UK network providers offer unlimited MiFi deals, and the price of unlimited data has come down considerably recently, making it a much more affordable option, especially as an alternative to fixed-line home broadband.
It varies enormously from a few pounds a month to amounts that will almost certainly exceed what you pay for your entire fixed 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 that 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 one 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, with one-month, 12-month, 18-month and 24-month options available.
Average speeds top out around 40Mbps, although you may get speeds faster or slower than this depending on the network you choose and the quality of the signal in your area. This really is a 'how long is a piece of string' question.