We do this- nursery is in DP's work. It's in central London and DP commutes with her most days- although I work nearby and do pick ups one day or if DP is ill etc. I was/am still breastfeeding and I could have gone in at lunchtime to do that if I wanted.
It's an excellent small nursery- it is contracted out but to a social enterprise, and the workplace is happy to subsidise it to run at a slight loss.
DD is 15 months and the commute is mostly ok- one train (18 minutes) plus a 15 minute walk or short tube if it is chucking it down. DP always gets a seat, they read books and chat about what they can see. He has flexed his hours a bit to miss the worst of the trains and to go home earlier- the day I do pick up it's usually quite a bit later as I have longer hours and she is noticably more tired on the journey and a bit grumpy. So finishing a bit early may be worth looking at. She hasn't had to be picked up sick once since she started so that hasn't been an issue. Being smaller probably helps with reducing the number of bugs going around.
One of the main bonuses for us is also that the little known tax benefit of having a workplace nursery is very beneficial- if your workplace runs the nursery you can salary sacrifice the entire amount (as long as it doesn't dip you below minimum wage), not just the 65/week- and that amounts to a saving of several hundred pounds per month.
The main downside is if you want a day off and them still to be in nursery- although we have been known to drop her and then have a date day in London which is quite fun. Oh and also carting nappies in every so often is annoying but DP could always get them delivered to his work or a nearby boots if he organised himself.
You do also have to be quite zen as even the most even tempered toddler will occasionally become very cross on the journey- it doesn't happen often- but fellow commuters are often more understanding than you might think.
And a sling is a lifesaver! Plus an array of toddler snacks.