I've got an unlimited 3G data plan with o2 in the U.K.
I checked the UPnP box and restarted Transmission and it gets to 8 kb/s then 0.

After checking UPnP (and restarting), check the status of the port again.

Port is closed.
I'm using the port number that the original reply suggested.

The problem is not the number of the port, problem is that your provider is not opening this port for you.

I know you don't remember your settings before, but could you describe how (which provider/connection) you used transmission before it broke.
And also could you describe the "breaking" process, how it happened.

Nothing has changed. I've been using o2 on 3g since day 1.
Every torrent I started downloading would lead to errors and turn MyDocs/Downloads into read only, then I changed the port number and now I can't seed.

Hmm ... I'm wondering if the port you are using is perhaps not in use by another client of o2 ...

Maybe try checking the random port option and restarting transmission.

No, that didn't help either.
Is there a way to completely remove all the traces of Transmission after I uninstall it so that when I reinstall it, it comes with default settings?

i have the same problem.
will hard reset help me? (reflashing etc. etc.)

Thought I'd share... the later problem posted here is a known issue with 3G networks in my country (ports are closed for packet data).

The weird thing however is that, changing providers significantly improves the network speed. Which suggests that it may not just be the port that's in question.

So I read some articles and found out that Torrent has this "Encrypt outgoing connection" option. After I enabled it the connection where I'm having slow transfer rates increased significantly.

My guess is that there's a system detecting the traffic on the provider's end. To relate that to this topic, I've noticed that Transmission also has this encryption option with the following settings:

Require Encryption
Prefer Encryption
Allow Encryption

I've tried experimenting with these and the Prefer Encryption option (which is the default I think) seems to have worked best, but connection still slows down every once in a while.

So my question is, are there other settings in Transmission that we can alter to better compensate for 3G / HSDPA connection types?

After a quick google search I found the settings file is located here:

/home/user/.config/transmission/settings.json

Delete the settings.json file or rename it and re-run Transmission.
A new file with defaults gets created.