Dabs and BT Shop are one and the same, BT having bought Dabs a couple of years ago. Thus they are big reputable company that'll still be there tomorrow. I know nothing of Saveonlaptops, but struggled on a quick search to find anything bad - which is a good sign. All that said, Lenovo is also a reputable company too and will stand behind their product.
OnePlus 2 review: Hands on with the £239 OnePlus Two '2016 flagship killer'