Your on NTL right? Let me tell you what you have (as Im on NTL MySelf) - you have 750k/128 COnnection - the other one they do is 1.5mbit/256 for £37.99 a month - that means your max Upload at the moment is 128K or 16KB/Sec.
You are confusing your download speed with your Upload Speed - Upload is the server your end sending information back to the people playing on the server, so with your connection you wont be able to hold loads.
Not trying to say you cannot run a server but just pointing what your true speeds are.