On what dongle can you get unlimited internet for only one month?

I want a dongle for when I go on holiday (Wales, UK) as I want to access the internet from where I'm staying. I need unlimited internet or as much as possible for only a month. I will probably be watching videos, downloading content, checking e-mails, visiting social websites like Facebook or Twitter. Is there a dongle that I can do this on without getting a permanent contract. If so can you link me to it