you sure it's a mains cable you are short of? As you should still have the mains cable that came with the TV, and the YouView box will have come with its own. But thats only for on/off and volume. Or if you want an all-in-one have you considered a Harmony universal remote? They're not cheap but come highly recommended. But box remotes can control both the TV and box if you programme them so. TV remotes aren't capable of controlling boxes. You should not need that setting for the Smart Standby and To Hell With The Planet modes though :-)
The passthrough option is better if you can do that, as the Y will reduce your signal strength, but with passthrough, you have to keep the YouView box at least in Standby, and if you use the Energy Saver mode, set RF Loopthrough On in the YouView Settings menu. Then you will at least be able to use your Magic Remote when not watching YouView, though your TV probably won't do the pause and rewind live TV that the YouView box can. Or if you just have a zapper box with no RF out, then get a Y splitter for RF, and a Male/Male aerial cable, put the Y splitter in the TV socket on the wall, and run one aerial cable to the YouView box and the other to the TV. If your YouView box has an Aerial Out/RF Out socket (the recorders do, but the zapper boxes don't, IIRC) then if you get a Male/Female aerial cable (Currys/Argos/Amazon etc) you can link the aerial signal onward to your TV, and use it as a TV. It doesn't sound as though you have an aerial to your TV any more.