Where can I find a pay as you go dongle without a time limit (UK)?

I'm a newbie when it comes to dongles but I am looking for one for my laptop. I'm an infrequent user and don't want to reply on finding WiFi spots. Im looking for a mobile internet deal that does not expire after one or two months. Can anyone advise me of a provider that allows you to use what you have paid for when YOU choose, without a time limit attached.