Frankly, in this case, doing anything the others suggest is a bit pointless, except for one thing.
The BT speedtester is telling you that the IP Profile is just over 14.2Mbps - and your sync speed is likely to be about 3% higher than this (when you sync at 40Mbps the IP Profile is 38717).
The actual download speeds you can achieve are 14Mbps, so you are fully utilising the existing link. This means that any requests to do more tests with wireless turned off, or with a direct PC connection, or whatever, are pointless. It is only worth doing that when the profile value goes up and your tests can't get remotely close to that value. For example, an IP Profile of 38717 should be able to sustain 37.5Mbps on a "speedtest.net" test.
In your case, you look to have been hit by the DLM process. In FTTC, the cabinet tries to run you at the best possible sync speed for the first 2 days, and then intervenes if you are getting large numbers of errors on the line - either by adding interleaving, or by limiting the speed, or both.
In my case, the engineer's JDSU showed I synced at full speed when he first installed the line. However, 2 days later, DLM intervened and added interleaving (so latency increased from 11ms to 18ms) and reduced speed by 4Mbps. I used the "think broadband quality meter" (TBB BQM), and this showed I was getting about 4% packet loss during the first 2 days - so presumably the line suffered lots of errors then.
At the time, you couldn't unlock the modem, so I couldn't see the error rates. Now you *could* unlock the modem to get these numbers (so is the one bit of useful advice). However, this is non-standard, because the modem is part of BT's service. You would be better off just reporting this to the BT help desk, and getting their help.
If you are < 100 metres, you should *easily* get full speed, and an initial JDSU speed of 40Mbps is a sign that you *can* sync high enough. The drop to 14Mbps is huge, and can only come from the error rates. Therefore you almost certainly need an engineer to help sort this out...
... but like ADSL, some self-help can be useful. Do things improve if you unplug everything else from the line (especially fax machines)? How about if you disconnect any extension wiring by removing the front-plate?
Get your own BQM here
My BQM on the day DLM intervened: My BQM