I don't think it's valid via Leicester but back when XC operated no HSTs and EMT had regular HSTs to Sheffield, I went via Leicester a few times. The first time I asked the guard and he was surprised I even asked, so after that I just showed the ticket and it was always accepted.
I'd buy it to Poppleton or Cattal though. Same validity, same price, but possibly less likely to be questioned.
A ticket I used to get a lot was a York - Nuneaton, this was (and perhaps still is) valid via a wide range of routes including I think via Preston (but I never did that!) or via Peterborough, to take the two extremes. And various lines in between. I no longer do this due to the ludicrous prices charged for walk-on travel. I would either get an Advance, get a lift, or not travel. I'm not being ripped off by XC.