Giffgaff with a one-off Gigabag costing £5 could suit your needs. It's 500MB, lasts 30 days, and allows tethering. They have some higher allowance gigabags as well.
If it's the Google Nexus 7 tablet (which has been the rage these last few days) you plan to buy, it is not yet clear if you can tether by USB cable from your phone. But if your phone has a Wi-Fi Hotspot option this should most definitely work with the tablet.