I would only use a mobile dongle as a last resort for broadband. they are at best a poor alternative.
if there is a phone line in the house at all then I would consider one of the normal providers that offer no contract broadband, of which there are a few.
http://www.broadbandchoices.co.uk/no-broadband -contract.html
myself out of the ones on that page would go for the be one that is a 3month contract as it is unlimited downloads. if your looking at the others then I'd stay away from the ones with only a 2gb limit on them.
Another option depending on your mobile phone and provider is seeing about getting a data allowance on your normal mobile and connecting to the internet through that on your computer.