Well I'm really surprised. When did it become law that trains had to have tanks (or did the railway companies decide to do it off their own back)?
I'm not sure it is illegal even today - just socially unacceptable in new stock.
The first retention toilet stock was class 317 working on St Pancras-Bedford, built in 1981.
But I think the last new dump-toilet stock was the class 158 last built in 1992.
That leaves most Mk3-based stock with dump toilets, although a few types have been retrofitted with retention toilets (as were the 159s from new).
Even now, refurbs do not necessarily remove the dump toilets.
It would be interesting identifying the "cleanest" routes in the UK.
The WCML must be one of the best, with 390, 221 and 350 stock fitted with retention toilets.
GW must be one of the worst, with all the HSTs and Sprinters.
That mine of information Wikipedia doesn't think toilet type is worth recording in its technical descriptions of UK stock classes.