Some call it "spending a penny". Network Rail call it "spending 30p".
London Euston is one of the few stations in the UK managed by Network Rail themselves, rather than the train operator of the area. They have this thing where they charge people to use the loo (30p, which I believe is standard across all Network Rail managed stations). I've not been to a station managed by a train operating company where they charge, though that could be a coincidence.
Nyways, the fact that you have to pay to get in should be a good sign that the quality and cleanliness of the toilet is of at least a reasonable standard. Aaaaand...it almost is. The toilets are typically clean looking, unless you're really unlucky and are immediately after the guy who doesn't flush (incidentally, if the person is within the vicinity still, Network Rail encourages dropkicking the individual back in to flush (this is a joke don't sue us)).
Flushing is handle operated, not sensor-based like some Network Rail toilets. As per usual, Points of Loo encourages the use of a bit of toilet paper to use handles on public loos.
Paper is plentiful, two large rolls per cubicle and there's always at least one cubicle free out of the past many visits.
For those carrying lots of baggage (it is an intercity station after all) there are extra-wide cubicles indicated by signs on the doors. Plenty of room for large bags. However, this sometimes means the paper dispenser is further away from you than normal, which can require a stretch.
Hand washing facilities are sub-par. Not very many basins with those depress and let go taps that are never running water for long enough. Dryers are those Dyson ones which, while quick to dry, are noisy and unpleasant to use. If you get the cubicles adjacent to the dryers, you get a draft and, in some cases, stray water flying in from under the door. The noise when you're not expecting it is also less than pleasant.
Finally, the smell. Yes it's a toilet. Yes I know they're supposed to smell bad usually, but not this bad. I can only assume the air freshening unit wasn't working as it was incredibly unpleasant to the point where I wanted to sacrifice my 30p and go somewhere else. Possibly the worst public toilet in terms of smell only I've been to.
So, overall rating is 3/5. Good if you really need to go and can't get somewhere else, but I'd save your money and go somewhere else. Train toilets are better.