Was meaning to add, have you long term plans to stay where you are (eg you own your flat, or on long term lease) as most landline contracts will be for a minimum of 12 months and some broadband deals have a minimum of 18 months (unless you pay a setup fee, and sometimes higher monthly fees).
Loads of companies offering deals, some with vouchers or cashback. If you can name your exchange (or find the code on samknows.com - 'exchange search' will give you a BT Openreach exchange code of 4 to 8 letters), it will be easier to determine which offers are available in your area. Not all services are available nationwide.
Like InickedaName, I've used the Three dongle in the past with 15 GB data each month (my contract was initially 7.50 and then increased to 8 quid) for the period October 2008 to Sept 2012, but recently I have been using hundreds (*) of GB a month with a Now TV box, for example watching the first 4 series of 24 over a couple of weekends :)
Also some video podcasts are over 1 GB per show, so an unlimited data allowance, if you are ever tempted to view films or catch up TV via the internet, is probably best.
I bought a Three router (like a MiFi unit but a MiFi is a rechargeable mobile unit, and mine is a mains powered one). That takes the 3G mobile signal via the dongle, and provides 2 ethernet sockets (for laptops, CCTV boxes, satellite/TV systems needing internet access) and gives a wireless hotspot for your home for Now TV, tablets, mobile phones (so as not to be using a fairly low data allowance).
Three offers a maximum of 15 GB for their standalone data dongles, and only 4 GB of 'tethering' (where the mobile phones gives a wireless signal to allow other devices to connect), so although they are the most generous with phone use of data (lots of 'unlimited' options from other networks slow down after 5 GB of data, where Three's 'All You Can Eat' means just that), it doesn't help anyone wanting to connect a laptop for more than a few GB of data a month.
(*) the line installed on Friday is my second internet connection, as I got myself a Chromecast for Christmas, which can stream video to the TV off my router. I expect to see my usage hit 450-500 GB in the next few months.