You can buy the just released Huawei E5776 4G mobile Wi-Fi device for as little as £49.99. You will also need to buy a one-month plan from EE which provides you with 1GB of data allowance for £15.99.
It is recommended that you give a 30 day notice as soon as you order the modem if you plan to leave EE immediately, otherwise, the rolling contract means that you will be charged for another month.
The plan also includes EE Film which allows you to download movies from this dedicated service without data costs, two-for-one cinema tickets, BT Wi-Fi hotspot and free Wi-Fi on the underground.
You should be able to unlock the Mi-Fi device and use it on other networks as well. Note that you will have to allow up to 28 days for delivery.
The Huawei E5776 is the only mainstream device on the market to support CAT4 speeds, which are up to 150Mbps downstream and 75Mbps upstream. Up to 10 devices can connect to this Mi-Fi and you can even connect a microSD card and use it as a mini-NAS drive.
Given how expensive 4G remains now, one may argue that it is better to stick to 3G packages that offer unlimited data allowance for now.