I'm sure the dealer recommends "at least" £20/m. I expect he'd recommend way more if he felt you'd pay it. These speakers RRP at about £900 and he thinks you're going to pay at least £240 for speaker wire? I assume this is where he makes his margin. However, if co.'s like PMC use bog standard Van Damme cable for their professional installations, why don't you give it a go and save a fortune? You could then budget for a better pair of speakers, if you wanted to spend another £150. Harbeth, who supply speakers to the BBC, recommend your starting point for cable as humble 79 strand, which is about £1.20/m from Maplins. Both of these speaker co.'s are leaders in their field, and know what they are talking about. Harbeth, in their professional demo's (i.e. when they want their speakers to sound at their very best) single wire, and leave the linking plates in. Maybe you could consider buying speakers from either of these brands, because you know they design their speakers using low cost speaker wire?
If you want to understand what thickness of standard copper wire is required for what length of run, try this link, and save a fortune.....
http://www.roger-russell.com/wire/wire.htm