This is getting more involved than I thought it would.
The socket in the bedroom is not a NTES, but a PCS whatever that is, and the phone and router lights annoy Swimbo at night so they have to be moved to the hallway.

Yeah, you're a bit knackered. Telephone extension cord is your only (legal) option, and they are unsuited to long distances.
Rubbish. The extension cable is just as capable of carrying the signal as the miles (possibly) of underground cable from the exchange.
I've (in a previous residence) used a 40m extension cable (ok, I was naughty and extended from the master socket) and had no negative impact on ADSL speed.
Anybody who has seen the insides of a BT Cabinet would know that a shielded extension cable is the least of your worries! 
EDIT: In fact, my Sky Router is plugged into the furthest extension from the NTE5 socket in my current house (up two floors, across the loft and down two floors) and there is NO difference. I have FTTC.
SKY Fibre 40Mb Up and 10Mb Down

Modem Status;
Modem
Modem Status Connected
Traffic Type:PTM
Line Rate - Upstream (Kbps):9995
Line Rate - Downstream (Kbps):40000
No standard phone cables are shielded, be it UG, dropwire or internal. And yes, I've been in more cabs that most... ...fortunately there aren't many Midland style cabs about now, and virtually all cabs built in the last 20+ years will be IDC. In cabs (and other joints), no matter what sort, both the E and D sides maintain their twist up to the connection, and the jumper is twisted as well to eliminate cross talk.
The problems with plug in extension cables are:
1) For voice and internet, attenuation is high - this is why you can't get a good quality lead longer than 5m
2) For internet, an extension cable is flat, thus not twisted, hammering the SNR
Guessing with figures you're getting, you're within 300m of the cab, thus the line can easily maintain the headline speed. If you were further away, or had an 78Mbps service, you'd start to notice potential improvements
