Hi OP. I see your dilemma, but I think you have to think long-term. In particular, what is the plan for educating your DC? If you’re in London or the outskirts, you would be looking at about £50k per year for the two of them - so that’s your salary gone.Then £200 odd k may not be that much for everything else, post tax, depending on the size of your mortgage and general outgoings.
As for your question, “How much is enough?” well, it purely depends in your circumstances, where you live and future plans.
It sounds as if your children are quite little now? I’ve been SAH for about 16 years. I thought I’d never go back tbh, because any money I would earn would be negligible in comparison to DH and we have 4 DC. I actually think it’s easier to have a nanny when they’re little, but when school and homework start to matter (particularly if they’re doing 11 plus for London independents) this is where a nanny falls short. You can’t expect them to be across all this.
I have many friends where one parent earns at least what your DH earns. They were fine both working until they realised that the nanny couldn’t help with the homework sufficiently and the DC were falling behind. That’s when the lower earner left work to be SAH.,
As a SAHM to primary aged DC, your day isn’t long really because you need to be mobilising at 3pm for school pick ups. It does make family life a lot easier, if you’re not exhausted and trying to cram everything into evenings. You wouldn’t be bored while they’re at school, because there loads to do, even with a cleaner coming in. You would be surprised probably. Also, there’s nothing wrong with taking your downtime during these hours because after 3.30 and weekends it’s full on and we are not machines. Also, you don’t mind if your DH wants to do his hobbies or whatever at weekends, so resentment doesn’t build up.
The time that you would probably want to return to work would be when there’s no more school runs - probably when the youngest one is in secondary. It might only be a couple of hours, but having DC come home on their own steam at 4.30 / 5pm does feel like a longer day than when you’re tied to primary school pick ups. The question is, how easy would it be for you to return to work at this point? It if you don’t want to return to the same field, what do you see yourself doing instead - eg maybe starting a business or restraining? Many women I know have a property development portfolio for instance? Some have online businesses that have been very successful; some are personal trainers or therapists who set their own hours? Having time out as a SAHP does give you the chance to reassess actually, or to think outside the box - and to reinvent yourself if need be 
Would your DH actually prefer you to be at home if not? You have to have this conversation obviously. Particularly if he travels a fair bit, it’s very reassuring to not have to worry about juggling the kids.
Good luck whatever you decide!!