It is possible to have some of 3's tariffs on Orange (more likely if you are out of your contract period though).
I called Orange and asked for my PAC code as I was considering moving network as Orange did not have the phone that I wanted. They asked what offers I had seen elsewhere. I mentioned 3 and they said I could have Select 500 (the Orange equivalent of 3's Videotalk 500) for £25. I didn't ask for the tariff.
I couldn't find any phone on any network that I really wanted and I didn't really want to leave Orange anyway. I decided to stick with Orange and have the Select 500 tariff but when I asked for the tariff they said that I would need to get a new phone and commit to another 12 months (which I had been told was not the case beforehand). I have no problem staying with Orange but I didn't see the point in getting a new phone just for the sake of it. So I am moving network (sadly).
Not sure why Orange wnt to tie me to a new contract which would involve them subsidising a new phone.