if your mobile phone has build-in modem (many of them have), you can connect laptop to mobile and dial a normal isp (same as landline), if you can find a isp with No-0845 number (someone said, tesco has), you can use your mobile free minutes. The connection speed is slow 9.6kbps, but it is an option.
If your phone and lappy have infra-red, connection is easy. Otherwise, you can buy a bluetooth adaptor for your lappy (it costs as low as £10).
Wi-Fi hotspot or landline is better option if you can find.