Why is my broadband running slower than its rated speed?

Your connection speeds can be affected by two factors that are within the ISP's control. The first is known as contention. If your ISP uses a contention ratio of 1:50, which is common, then the ISP has provisioned enough capacity for you to obtain the full bandwidth consistently all the time as long as you're the only person on your block of connections that's using the Internet. There are 49 other people on your block of connections in a 50:1 contention ratio. If 4 other people are connected at the same time as you, then there are 5 of you fighting for the same bandwidth. ISPs justify this by explaining that, whilst one person is downloading a web page (for a second or two), the other 4 people (or 49 in the worst case) are probably just reading the web pages that they've already downloaded. However, if 5 of you are all downloading large files that are going to take more than a second or two, then you're all going to get one fifth of the maximum bandwidth. That's why ISPs now have to advertise their services as "UP TO 1Mbps" or "UP TO 2Mbps", etc.

The other issue is how much capacity the ISP's network has. If they don't have enough DNS servers, mail servers, web servers, etc to cope with the amount of customers they have, then all of their services will be slow.

You can get an uncontended service but this is VERY expensive and almost always only within the reach of businesses. One company who I have a feeling can provide an uncontended service in the UK for consumers, however, is Pipex. They've dropped their prices recently but Pipex has traditionally been a "premium brand" company - in other words, you'll get a fantastic service off of their well-specced network and brilliant support and they'll have the happiest customers of any UK ISP ... but it costs you! You could check out their web site but, if they offer an uncontended service, it will be expensive.

Even if Pipex don't do an uncontended, or low-contention, consumer service and I was imagining it, it would still be worth checking them out. Ignoring cost, Pipex is widely regarded as the best.