Is sadly doesn't sound like his business is going to be viable even without Covid (based on the amounts of £ you have said, and the long hours he works for that) for you to have a second child, so at some stage, I think you need to have that hard decision as a couple.
That being said, the double childcare fees is a limited, and short term problem, and there are solutions which people have mentioned. I imagine it's viable if you want it to be.
Re the HG, I didn't have this, but I had a different pregnancy complication in both of mine that rendered me mostly housebound for most of my pregnancy. I had a similar age gap to what you'd have, and in hindsight, I'm very glad I did it when I did and didn't wait.
Also, with parenting during lockdown, with two small ones that have (during the year of this) spanned the ages of 11m - nearly 4, younger is better when it comes to being stuck in!
An under 2 year old is more likely to be content hanging round the house/garden, whilst you cradle your bucket, than a 2-3 year old who will want to go to the park, will need more interaction with friends. Having done mostly housebound pregnant parenting with a 1+ year old, I have no idea how I'd have coped if my first born was older.