The law already differentiates, it's just that across the whole Theft Act, the same sort of sentencing levels are used. What you possibly may wish for instead is a push for harsher sentencing across the board, but that's a whole other can of worms!
That's not (as ever sadly with your posts) a very nuanced view at all, and especially from one who works in this area. You really do need a bit more imagination than that.
Theft isn't some naturally defined thing that must have a parity everywhere. If something doesn't work, you change it in the hope that might shift the balance slightly.