WeBuyAnyCar is part of BCA, British Car Auctions, BCA have typically taken trade-ins from the dealers which BCA then sell at auction. BCA are cutting out the dealers acting as middle men by approaching the owner/seller directly, bypassing the customer trade in process.
I'm warned that the WeBuyanyCar offer is a baiting tactic, when they have inspected your car, there may be a heavily revised offer to you