You can get a SIM only for mobile broadband from Three, loaded with 10GB of data and with a one-month rolling contract for a mere £15 (a micro-SIM version is also available).
Oddly enough, the mobile phone operator has priced its 5GB offer for 99p extra with the 1GB one available from £7.50. At £1.50 per GB, this is the cheapest PAYG/short contract data bundle we’ve seen on the market.
In comparison, Vodafone sells a 2GB 30-days Data SIM only for £15 (with an extra GB costing £15), O2 has a similar offer for £15.32 (albeit with unlimited Wi-Fi) and T-Mobile/Orange data SIM-only costs £15 for 5GB.
It does require that you have a compatible/unlocked dongle/device though and that you use it in an area with a good coverage.
Also you will get a better deal if (a) you want a longer contract (b) you can afford the £25 The One Plan which offers tethering (which requires a smartphone with a tethering option).
Three also sells a 24-month broadband offer with a standard dongle for £16, a MiFi device with 1GB pre-loaded for £50 or a standard dongle for £30.