You won't get anything better from any other ISP if you find out that:
1.You don't have filter
2.You have filters in wrong places
3.Your phones are causing interference
4.Your alarm is causing interference
5.Your internal wiring are of low quality
So as suggested before you have to first determine those conditions. Being so close to the Exchange points out to an internal problem - most of the problems are internal.
If you can certainly rule all these out, then get some Telecom/Chorus technician to check the connection. But simply changing ISPs won't help if the problem is inside your house.
+1 You need to do this before making rash changes.
It could be something simple like a bung filter (you do have filters don't you?). Unplug everything and see what happens, then slowly plug everything back in again and see what happens each time. Note that if you have a house alarm you may need to disconnect that from the phone line to check that's not the problem (common IME)
Beyond that I would be making some polite calls to the ISP.