Ok, so I've solved the slow download issue after a bit of messing about, and it was quite illuminating.
From a bit of googling I discovered that earlier in the year there were some slow patching issues for some people, and the advice given was to make an entry in the HOSTS file to associate a particular IP address with the hostname cdn-patch.swtor.com. The reason for this is that there was a problem with one of the IP addresses given out by the load-balancer that was used for swtor's content delivery system, hardcoding this IP address in the HOSTS file allowed users to bypass the load-balancer but was advised as a temporary fix with the entry to be removed after use so that the IP that had been given out wouldn't become overwhelmed.
I tried that as a fix but the slow downloads continued at the same snails pace as before.
So I researched different IP addresses that had historically been associated with the cdn-patch.swtor.com hostname and started going through them one by one as an entry in my HOSTS file to bypass the load-balancer and speedtest the results.
The IP address that was given out as the old slow patch fix was one of the slowest (50KB/s to 100KB/s), and was coincidently also the IP address that was always given to me by the load-balancer when I had no entries in the HOSTS file to bypass it.
A few other IP addresses were a little faster (300KB/s to 500KB/s), but then I came across an IP that was on a whole different subnet to all the others and I tried it out in my HOSTS file.
Using that one I hit 3MB/s immediately, and although it's speed has reduced a little it's bouncing between 1.6MB/s to 2.74MB/s quite consistently.
To check that it wasn't just a co-incidence I removed the entry and tried again, once again I was given the slow IP by the load-balancer and my sub 100KB/s speeds returned.
I re-enabled the fast IP in my HOSTS file, flushed my DNS cache (Had to do this ALOT) and restarted the launcher, immediately my fast speeds returned.
So, my conclusions:
Either EA have a broken load-balancer, or they may be purposefully prioritizing traffic from subscribers IP's to the faster content delivery systems and leaving a lower priority machine for non-subscribers.
When they post advice on their forums telling you to run ipconfig /flushdns and try again what they are actually telling you is "clear your DNS cache and hope that the load-balancer gives you a nicer connection to a better, less populated patch system" and makes absolute sense, but is unfortunately ultimately flawed, because whilst you can flush your local cache via this method, it ignores the fact that some ISP's themselves cache DNS requests, so you will get the same IP address over and over despite the fact that the load balanced system should give you a different IP.