There no such thing as being 100% accurate, because different speed testers have their own way of doing things. One particular difference might be your locations, as to the server route the test uses. In England, there are three main route servers- London Docklands, Birmingham and Manchester.
If you use some ISP recommended speed tests, then you might be able to route via your own requirements. One site is http://www.speedtest.net give you the option, and is recommended by TalkTalk, when they do tests.
I use a number of speed test downloads for making a comparison, but I find that Jack Dinn's speed tester fairly good, because this allows you to do various other checks (trace/mtu etc), if the connection and speed requires checking.