Why doesn't Simon subvert Gemma's scheme?

Gemma manages to blackmail Chris and Simon to recover the money Simon siphoned from their accounts. But she also needs the housing project to be completed and succeed in order to get paid off.

Chris has the greatest incentive to go along with it, the conflict of interest in council membership and investing in the project would get him in trouble, plus he would lose his share of the investment which for the most part Gemma isn't trying to deny him.

So what motivates Simon to continue with this? He's the one who has to keep working on the project while getting nothing out of it, plus being stuck with a pregnant girlfriend he has to support without any money from her rich dad.

If he wanted to get revenge, he could submarine the development project and keep Gemma broke. Sure, he would be exposed for the mortgage forgery, but he might even skate on that. I don't know what British law would be on this, but a lawyer friend here in the US tells me in a domestic situation like this, the victim of the crime is mostly the lender, not the spouse, and they're probably made whole by the sale of the house. Community property laws in marriage complicate the criminal fraud involving the spouse, since you can't steal money from yourself.

Anyway, Simon could fuck Chris *and* Gemma by simply bailing or intentionally wrecking the project. Gemma doesn't get paid and Chris loses his investment. Simon is probably no worse off.