At the moment stick on plates are ILLEGAL even thou they conform to the whole BS standard. How ever the plod were told by the government to have some common sense on these, the history of these come from the S-Type Jag where there was not a flat panel so they were the first to have stick / painted plates on the bonnet and it was them that got the government to use common sense, HOW EVER, any other vehicle running them can come into trouble if there is a better place / style to have. I had a VW Baja which was all round so had a sticker plate and no problem with the police even after a routine stop check .
As for position it should be visible but at the moment i do not believe there is a legal height for them. your 'Safest bet would be on your grill as if you put them on your vents you could be bollocking for preventing a copper doing his job or similar if you have your vents open and go past a speed trap.
Have you thought about making some thing which could clip onto your fair lead or two LARGE 'P' clips to go round the tube bumper and then bolted to the plate as normal maybe with some 2mm steel brace so it doest brake in the wind???