My understanding (FWIW) is that it's perfectly okay to have toiletless coaches and perfectly okay to have toiletless trains - however if there is a toilet on the train then there needs to be at least one DDA one (so you could have an unrefurbished 153 permanently coupled to a 150/156, just not use the 153 on its own).
Or, another way to look at it, you could have two 153s coupled together okay as long as one has a DDA toilet. But if one was separate then it'd have to either have a DDA toilet or have *no* toilet.
(yeah, I realise how daft it is that no toilet is better than a "non-DDA" toilet under the rules)