Have you tried disabling the Belkin router WiFi and testing with a computer strictly connected via ethernet?
As suggested, connecting the router via the test socket is also a good idea as it rules out any "internal" wiring issue.
Without testing in that manner there's too many variables at play to sensibly work out where the issue is.
If that yields the same results then it's really got to be down to one of the following:
- Faulty router/modem
- Some kind of fault that is the ISPs responsibility
- Actual limitation of the line (You can maybe have a look and see what the BT Checker estimates the line should sync at [not conclusive but might be indicative])
In terms of recommending an ADSL router, I think the Billion 8800NL is particularly good on longer ADSL lines so that might help quite a bit on the WAN side of things, however its WiFi performance is poor so I'd use it as a modem only and bridge it to something a bit better (like a ASUS-RT-N66U).