It is the distance from the exchange where your line terminates that is important!
However... wI-fI probably will decrease your speeds, and more importantly add latency (low speeds don't necessarily mean high latency
btw) so either run cat 5 cable (cheapest option - and best capability) or use homeplugs (there was a post today recommended devlo brand.
Secondlly - how are you connecting to the BT line? Are there any extension lines in the house? Are you connecting through them? Do you have the new NT5 split junction box where the line comes into the house? If yes, remove the faceplate and connect the router to the direct socket behind it (you don't need a filter for that, although you will need an rj11 to bt connecter adapter - but if you haven't got one of those, use a good quality filter - such as the ones BT supply They cost about £6. If that improves things, then there are problems wiuth the internal house wiring. Besy solution is to replace the house telephone wiring . Best solution is to replace it, but if you can't, next best solution is to put a fuiltered faceplate on the incoming line, but that does mean that the router has to plug into the faceplate. (You can get them from Solwise) I am assuming that you have already filtered every other device connected to the phone line - extensions, skyboxes, fax machines etc, etc - with good quality filters - it only takes one duff one.
Routers do make a difference - the most sensitive used to be the BT branded ones (well they would be - BT wanted as little trouble as possible with them!) but I don't know if that is true now. Draytek are reputed to be good, and did do special firmware releases for weak signal areas, but I think they have incorporated that into the mainstream firmware. However I have no comparative information on them
So in order of priority
Ditch wi-fi
Check filters
Check extension cables
Consider a new router
Incidentally, to get any changes on the broadband aspect of your line, you have to go through your ISP - you can't contavt BT direct (they are not your retail supplier) However, if you have noisy voice calls (and BT are your telephone supplier) you can ask thenm to investigate that, and any improvements may improve ADSL - but don't make something up, because if the find no fault, you can be charged!)