Your download issues may be with your line (do you live in a rural area?), or simply that the places you're downloading from is heavily subscribed (i.e., may not be Tiscali's fault, and won't be improved if you switch to BT or anyone else).
Virgin are pretty good (and if it is the line that's the issue, then this will sort it, since they use their own line), the only issue is that they have an idiot of a CEO right now for Virgin Media, who wants to turn the internet into some controlled environment like cable TV.