You may be correct to say that colouring books are helpful to "de-stress" you may be correct to say that £900 for 300 books is cost effective.

However I would say that individuals should buy such things out of their own salary. I would also add that some of these these books retail at £16 each.

https://calmoverchaos.co.uk/collections/all

I would also say that this is an abuse of government funds.