As someone who regularly handles £50 notes, I can say that this is very true. Larger denominations are much more well kept, but this is likely due to the fact that fewer people will have handled them, so there's less chance for them to get crinkled up and damaged. And the fact that it's usually businesses that hand them into banks and post offices as opposed to individuals making deposits.

Lower denominations like £5, £10 and £20 notes are handled by the public a lot more frequently, so there's more chance for those notes to get damaged or worn down.