Computer magazines rate this company as a "Best Buy" for best coverage of the UK and for price. I use one of their USB modems on holiday in the UK.
Go for Pay As You Go. I top up with £10 which lasts for a month. I use about 1.6Gb a week and the you can get 10Gb for £10.
Three - Mobile Broadband - MiFi
You don't need a contract with them.