Uhhh, it goes through a faceplate and goes up a wall as high as roughly 2 full ATX PC cases and bends around like 10 other cable all the way to the Plusnet box.
The cable looks like it's been used a hell of a lot and is grimy and is really thin, maybe about 1mm thin. I'd say the cable like 1.5metres.

Sounds like crappy wiring. But your line is pretty much the same thing I work on. I just don't know what the standards are in UK. But if your wiring doesn't look good chances are it creates problem.  The things to do is have a dedicated telephone jack for your modem (Without phone plugged in) that way the phone doesnt mess up your internet. You need a wire that goes from to your provider box to where you need your modem. 

The thing with copper cable is that distance matters a lot. So if you're far from your fiber it reduces your internet speed. There is no real way for you to know where your line comes from unless you're a tech or know one =P Some neighbourhoods are worst than others. It really depend on where your internet comes from. 

Also, you have a modem and a router plugged in? Have you tried to test your computer plugged directly in the modem? Not through the router. This might indicates if it persist that the problem isn't on your side...

My internet comes from the Rhyl exchange (which serves over 1,600 houses/shops) which is physically 1mi away, however I believe the wiring goes north, making the wire about 1.5mi long.
Also, not sure if its completely irrelevant, the cabinet I'm connected too (in my case Cabinet 9) has a direct fibre connection to it, allowing FTTC.

Edited April 3, 201610 yr by Starmix

If the cabinet has Fiber then it makes it even closer to you so distance isn't your problem. As I said best thing to have it a dedicated phone line for your modem that goes to the BT box. When troubleshooting, i always use my meter in the customer's jack to check how my signal is because sometimes a faulty modem could cause your speed to decrease. Have a technician come to check your speed and tell him about that and he'll probably help you out (Ask him directly if he thinks the wire is bad or the modem)

Edited April 3, 201610 yr by LeGeorgettier
Cuz I dunno owl to wright

I'll probably replace the wire & router soon anyway.
The router's top plastic is starting to go yellow and the cable looks like its not going to last much longer.

That's your best option! I would still try to troubleshoot with one device plugged in the modem to see if it helps! then you could only change your router if it works fine.

I'm going to have a good look at it all tommorow.

KCOM user here, still waiting for that lightstream. Sorry for bump, just announcing so, because if I don't have fast WiFi, GTA V is impossible to even start up because of yuge update.

Also, Starmix, subtly creeped out by your avatar choice. Guess I have great expectations for the newest episodes of Family Guy.

Nah, it's from one of the old ones.
Also, turns out Plusnet or BT is purposefully limiting the connection.