Its simple I think.
You get yourself a new PAYG Three sim, stick it in your MIFI and activate it, then you go to My 3 and you top up £15.
You then convert that £15 to a All in One 15 add on.
This add on allows unlimited data AND tethering in a phone.
It should also work in a MIFI, mine does I got a MIFI on £15 monthly rolling contract for £39.99. But I cancelled the contract and use the PAYG sim instead as I dont have to watch data use.
Failing that you could buy a cheap android phone of Ebay and use it to thether, or share wifi hotspot on if you want to be ultra safe.