If you're on a cheap(-ish) broadband package, the service will probably have a high contention ratio. This is typically 50:1. (i.e. you'll be sharing bandwidth with up to 49 other users. If they're all online at the same time as you, your download speed will suffer. (To get a decent broadband service you really need a maximum contention ratio of 20:1, which is available through business broadband packages).
Further, things like the BBC's iPlayer require an extremely large amount of bandwidth. ISP's detest it and they've tried (unsuccessfully) either to get it shut down or to force the BBC to pay towards the massive costs of upgrading their infrastructure. In the meantime, as it becomes more popular, everyone's download speeds will suffer.
In order to share out the available bandwidth as fairly as possible, most ISPs now 'throttle' the bandwidth of users. Some, like Virgin Media, only apply 'throttling' to high volume users. See here:
http://www.virginmedi...raffic-management.php
Other ISPs use a throttling policy (applied to all users) based upon the time of day and the type of service being accessed. Normal web-browsing and email only suffers minimal throttling (if any) but peer-to-peer services (e.g. Limewire) and streamed video (e.g. YouTube and iPlayer) can be throttled back by up to 90% at peak times.
The only way you'll get consistent really good download speeds is to move to a country where there's an excellent infrastructure of fibre optic cables to support such speeds. (e.g. South Korea). In the meantime your only alternative is to seek an ISP which offers a good contention ratio and minimal (if any) throttling. Expect to pay at least £30 per month for such a service.
Chris