If you can check some of the websites from mobile networks, you should get a clue as to signal strength in areas you visit, to chose which network seems to be best.
Sometimes you may see deals on for the USB modem (dongle) at prices as low as 20 quid.
Ebay has sometimes had sellers of SIMs for Three (a network I use for a dongle) where you can pay say 8 to 9 quid for a SIM pre-loaded to allow 3 GB or 3 months, whichever comes first. 90 days starts on the first day of use, so best only to plug in when absolutely needed.
With their online usage monitor, you would be able to see how much data allowance was left each week and get a fresh SIM before you hit 90%.
3 pounds for 1 GB is a fairly reasonable deal (for PAYG especially) but the initial cost of the USB dongle will be very variable depending on source.
I just spotted Superdrug selling a dongle (with 3GB SIM included) at 34.99 (and laughed when I saw "£57.36 per 100g" !)
PC World has some dongles for 19.99 at present, so you could get one of those and spend a further few quid on a Three 3 GB SIM via Ebay.
If you find your usage is low each day, it would perhaps be easier not to switch from dongle to wi-fi, but just up the 3 GB - it works out at about 250 MB a week (and some say they don't even use 250 MB a month, at least on a smartphone). Use it up with BBC iPlayer to download some TV shows - after all, the SIM comes with 3 GB so may as well use all 3 GB !