It's most likely because you're in the UK.
I should be adding a UK and Asia server by the end of the year.
My bandwidth provider has extensive peering to the UK so the tests here will still give you a very good idea what your speeds are to most servers (since many major sites are hosted in the US). When the data has to travel nearly 6000 miles, across the ocean... over about 30 hops (I pulled a trace on your IP) you'll never be able to max out.
Unlike some other test sites testmy.net does not rely on donated servers. For quality control I like to maintain everything myself. But because of that fact there is a heavy price that I have to pay to host such servers. I recently upgraded the main server, it's way over powered.. just how I like it. I just need to get the extra money together to pick up a few more servers.
I'll most likely be picking up a European server first. My host has a datacenter in London so that's most likely where it will be hosted... I should have that one nailed down in the next 30-60 days, so there's something to look forward to. Keep checking back, once that's up and running it will be very apparent... notices will be shown to all users that can benefit from that server and traffic will eventually be automatically routed to the closest server to the end user.