Some years ago I'd often be travelling from Cardiff to North Wales on a Sunday morning, which generally requires a change at Crewe. The coast train was due to leave ~7 minutes after the ex-Cardiff service: an easy connection on a Sunday, but less than the 10 minute minimum. As a result, I'd be given a 53 minute wait on my advance ticket.
On one occasion I asked the guard of the unofficial connection if she'd permit me to travel. She declined, and asked me to wait for my booked service (as she was perfectly entitled to).
On the next (and last) occasion I decided to feign ignorance and boarded the unofficial connection. (I was prepared to disembark at Chester and wait for the booked service if asked, or face the music otherwise). As it turned out, the guard on this service made no note of the fact that my reservation showed 11xx instead of 10xx, and I got home an hour early.
Moral of the story: ask and you may receive, but be ready to accept the consequences of refusal. Corolary: Forgiveness may be easier after the fact, depending on how strict/particular your train's staff are.