However, 3G coverage is still limited, and it's unclear what happens if you move out-of-range, e.g. does the dongle just stop working, or does it continue with a slower protocol? The slowest option I expect would be GSM from my phone (bluetooth between Mac and phone), but I expect this would be charged per minute and outside of my inclusive minutes if to a 0845 internet access point.
The orange data cards I have used simply fall back to gprs if the 3G signal is not present. However this is quite different from dialling an ISP as you suggest. Your access will be through the Providers data access point not dial up to ISP and as such will be charged based on data transfer not time. Theoretically you can still dial up a 0845 ISP via a mobile phone but you will achieve data rates far slower than even GRPS is capable of.
My advice is to forget the 0845 ISP and just get a decent size data bundle with your contract. Don't bother with a USB data card and just get a 3G capable phone as a free upgrade and use bluetooth to connect to that. No need to pay the line rental for a data card/dongle as well as your mobile phone contract. Don't forget to tell your provider to enable your phone's sim for full data access as not all are enabled for full internet access by default.
Regarding wireless hotspots...The BT cloud/Purple Patch ones don't work very well via Linux without some workarounds (that have the happy coincidence of getting you free access) Skype Zones is very cheap compared to usual hotspot rates (6.5 euro a month for unlimited access) and has a reasonable list of compatible hotspots. However it requires the Windows only client software to work.