I hadn’t realised until now that it states in my contract I need to let them know
You are supposed to know what's in your contract.
If I was your employer and you were not doing aspects of your role because of pregnancy and then found out you had a side gig on top of a full time job I think I'd be pretty pissed off tbh.
Of course it's really stressful being on a disciplinary, but if you have done something to warrant it unfortunately, pregnant or not, it has to be done.
You said in your OP the letter said you will get a warning if it's upheld, so you are not going to get sacked. Best advice I can offer is hold your hands up and say you made a mistake, make it clear the other role won't interfere and is desk based and aligns with the same health and safety constraints as your current role does. And if you get a warning, well, you breached your contract of employment. As a business owner and employer, your DH should also have known that working for another employer outside of a full time contract usually needs to be disclosed too.
Usually warnings only go on your file for 6 or 12 months so if you otherwise have a good track record it'll cease to be a problem as long as no further issues.
Whether or not you're entitled to keep your company car during maternity leave will depend on the terms on which it was given to you, so you would need to check your contract/company car agreement and check with HR. Where I work, there is a clause in the agreement that says if you are going to be off work for 6 months+ (which covers things like maternity leave, sabbaticals/career breaks) you may be asked to return it. But then obviously if you return to the same role with the same benefits they would need to provide another one.