Michael;
I wouldn't concern yourself about throttling (reduced bandwidth) on the cable network. In order for that to happen to you would have to download a darn sight more than 250Mb per day.
Admittedly in my area (Blackpool) throttling does go on, but it's only at certain times of the day and only on the "big" down loaders. Anyway, If you read the VM's terms and conditions it will give you an indication of what is acceptable.
You're worrying too much - there's no need, just enjoy the service
Virgin Media 1 - Tiscali 0
Si thee