I suppose by asking questions I'm trying to figure out why the OP's parents are so determined for her to have a termination. OP says they don't provide childcare. They don't clean OP's house or pay for a cleaner. They provide money which enables the OP's DH to drive an expensive car. OP's DH works rather have an expensive car than support his wife and children, and is vague on details when it comes to his finances.
OP's parents seem to be worried that OP neglects her eldest DC in some way. OP says she doesn't neglect either of her children and is capable of looking after both of them. OP's second child is 18 months old I think.
OP's husband won't get a vasectomy but wants OP to have a termination so he can keep his expensive car (£300 per month) and contribute approx £170 a month to his parent's household where he sleeps 3 nights a week. He is also paying back a loan to his parents of £100 a month which was to fix the front door of the house, and is nearly paid.
OP's parents subsidise OP and her DH about £300 a month. They also paid the deposit for the car so that OP could have a better car. But OP drives a banger as the new car is DH's and he takes it away half the week, although OP is allowed use it when he's home.
OP's DH has a take home pay of about 2100 a month, OP thinks. If he worked nearer home she thinks his pay cut would mean an annual salary of about £17000 instead of £32000.
Posters have suggested returning the expensive car to the dealer and getting a smaller car loan. Possibly £100 per month, freeing up £200. As OP's PIL are nearly paid back, that's another £100 per month freed up, giving £300 total, so OP and her DH would be able to provide for themselves and their children without financial support from OP's parents. They also have a SKY package of £68 per month (that's a fair amount of food) and a mortgage of £500 a month for a 4 bedroom house. OP doesn't really know what her DH spends money on and attempts to ask about anything lead him to get defensive and argue.
Whether enough cut backs could be introduced to support another child or not I don't know. I don't know how many years are left on the mortgage. I don't know why OP's parents think she can't cope currently. I don't know if DH comes home after his 3 days away and does everything for the house and DC (seems unlikely).
It's not a straightforward situation of I can't afford another child. It's a question of choices to be made for what some people might consider luxuries. The present DC aren't getting much use out of Daddy's car either.
I probably wouldn't want another child with that kind of DH, but it's a different thing when your pregnant already. Some women find it hard to come to terms with having a termination, especially if they would like another child.
OP, I hope that that summary is fair.