You can look up the royal grading the UK uses for classical pieces and use that.

A single difficulty score is a bit difficult (hah) as a piece will have many technical dimensions to it, and ideally you're comfortable with most of it but a few dimensions push you out of your comfort zone.

I think these standard grade scores attempt to give some kind of loose ordering to when you might want to tackle the piece.

I'm actually from the UK. Is this the grade system people normally refer to - like they say they're grade 8?

I meant more an algorithm to determine the difficulty. Things like tempo and how much your fingers would need to move. But equally, I could probably just cherry pick some easier pieces far quicker