Nothing you have written conveys why you should leave the father of your kids.
Meeting a new someone who makes life seem exciting cannot be compared to the man who you are raising children with and who is keeping a roof over your head and keeping everyone fed and dressed, whilst allowing you to be a SAHM and now earning what is quite frankly just pocket money.
It would be a real kick in the teeth to leave him now. Usually the baby, pre school and primary school years are when you hunker down and grit your teeth as a couple and just survive as a partnership, as a team. Then you emerge after that stronger and more stable than before to enjoy the rest of your lives together....... my guess he is in this mode whilst you have too much time on your hands so can find another love interest. This will not end well. Boyfriend will not stick around once real life gets in the way and kids need to be factored in.
Fiancé will lose his shit over the fact you've been unfaithful whilst he has been a mug and enabled you the time to be so!!!
Get rid of the non sexual relationship guy right now. End it. Get back to your fiancé and your kids. Take a holiday. Talk it over.
I think you need to be clear why this relationship is over before you walk out the door. You definitely should not just jump into a relationship with someone else tho.
And yes. Postpone the wedding.
Unless this is the pre wedding jitters? (We had these. I sympathise. We had couples counselling to get over it)
Have you thought about relationship counselling? Will help you understand whether the relationship is over or if it is just something else that could be resolved.
We had 1 counselling session together which made me understand it wasn't me but him that had stuff to change. I had to learn to react differently to his reaction to his shit but it wasn't my problem to solve.
Just this resolved so much tension!!.
He did go on and have a session on his own and then a few Years later he made some changes to his life. He is much happier himself now as a result. Which has a huge benefit to me!!
We've been together now about 15 years and married for many of them. However we nearly didn't get married. I did walk out W-day minus 5 months.
However we had those sessions, those realisations and then we had an amazing restorative holiday together with a group. Made me re-remember why I loved him. At W-day minus 3 months I bought the wedding dress and it was all back on!!!
So. Be very clear with yourselves what is going on here. And don't cheat on him with someone else because you are bored of him. If you are that bored of him - and this cannot be resolved - then leave. I'm sure life on £100 per week with 2 kids will be equally boring and most likely a tonne more work so I'm not sure why you would exchange your current life for that!!!!! You certainly would not have much time for a boyfriend unless you expect the father of your children to keep you as well as them after the relationship ends.......