Many (most? all?) tablets don't have the drivers to support dongles.
However you can probably find a (free) BT hotspot without too many difficulties anyway. (There are 5 million of them in the UK). Your BT Home Hub, for example, transmits a public-access signal (as well as the 'private' one which you use in your home); the same applies to everyone else who has BT Broadband. (You can turn the signal off but most people probably don't do so)
To see what the coverage is like in your area, enter your postcode here:
http://www.btwifi.com/
Don't be deterred when you only see a few mauve 'wifi' markers. They're just the 'premium hotspots' installed by BT. Anywhere with a (greyish) blue colour actually has a signal. Click on the '+' button several times to zoom in and find out where all of the BT Home Hubs are in your area.