Yes, in 1976 we used to use our bath and washing up water on the garden and our allotment with no ill effects whatsoever (except that all the plants were much cleaner than normal
) We've done the same thing several times on odd occasions since and it's fine.
The hose ban does make even this more difficult though as using a hose to siphon water from the bath is far easier than faffing about trying to fill buckets in water which is just a few inches deep.