I got a three huawei mifi from store.three.co.uk.
It cost 50 quid for the mifi and 1 gb prepay sim card that I didn't really want as I already had a sim card.
put in sim card, plugged it in to charge, chose network on my laptop, put in preset password, and it worked without any further fiddling.
lots of menu options to play with, esp to do with port forwarding and the like. but for what I'm using it for I don't think I need that. maybe if I want mobile SMTP one day(?)
they didn't send a vat receipt which is annoying but not disasterous. I eventually found an email form on their website to ask for one.
cool to get on train, and almost seamlessly get a network connection. marred by the fact that my mifi is lower in my OS's priority list than a nearby broken BT Openzone hotspot, so it went there first.