??
If someone was charged with an indictable only offence it could only be tried in a Crown Court and even with a not guilty plea it would still be heard in a Crown Court.
You can not choose to be tried in a Magistrates court with more serious cases.
I have never heard of murder, robbery or rape to ever have been tried in a Magistrates court at the request of the suspect.
It's a serious question though. You're saying that the law should be changed so that 25p thefts are not triable either-way. I think it's fair to ask where you think the line should be - even if it was discretionary there would still need to be some sort of guidance.
I would argue otherwise, Magistrates inconsistently sentence people and regularly handout supervision orders and the like even in the face of a mountain of previous like for like convictions.
At a sensible guess, maybe a limit of £1000 ?
Not surprising that the Peelers caught him.