Again it's all down to the quality of the cable used. RJ11 just refers to the plug on the end of it.

Give it a go, it's only going to be a 2 minute job to plug the cable in and trail it across the floor then stick the router on the end of it. If the ADSL synch speed drops significantly then just return it.

Although from the Returns policy on the Maplin site:

Therefore if for any reason you find that your recent purchase is not quite what you wanted. Then within 28 days of purchase, Maplin will replace, offer a credit note or refund your unsuitable purchase to equal value. This is provided the goods remain in their original condition, unopened and unused and accompany all packaging an accessories they came with.

So maybe worth clarifying with them in shop whether you'd be able to return if it just under-performed.

Get yourself some home plugs. Some of these are also apearing with wireless built in.