If I'm reading it correctly, you have an upload speed of 1 megabyte (8192 kilobits) in the UK while your download test indicates 5 megabytes (40960 kilobits)? Are you sure that you're not mixing up bits to bytes?
Try this site, (http://www.speedtest.net) and use the world map to select Singapore if you're currently in the UK or UK if you're currently in Singapore. The results there are listed in Mbps (megabits), and selecting the alternate location will allow you to test available bandwidth and latency over transcontinental links. Don't worry if your speeds don't match your ISP's advertised speed since those are usually burst not sustained.
Here's my test results.
Now for the reason you're only 200-300kbps is due to your current conditions. the Slingplayer has a two-way communication with your Slingbox. Its algorhythm will adjust the encoding to give you the best picture with the least amount of bandwidth possible under your current network and hardware conditions. If you'd like to see your current statistics then check below.
Hold down [Alt]&[Shift]+[i] for the Slingplayer Desktop
For the Web Slingplayer, hold down [Ctrl] and click on the picture.
If you still believe that you should get better, then you can go into your Slingplayer Options (Alt+O) and manually tweak the settings in the Encoding Tab to adjust the stream parameters.