Yes, mortgage holiday! I am so glad I found out my mortgage company did payment holidays. We took the maximum 6 months. I've no idea how we would have coped otherwise. Lots of mortgages do this - just ring and ask if they do it, you may be pleasantly surprised.
You say you do a unique job and feel bad about leaving your company in the lurch. Earlier on you say you need to do the job for the money.
While I'm sure both of them are true, it might help to have a think about what your priorities are. While you are actually at work, it's easy to feel needed by them. But when your baby is born, your priorities may totally change, and work (or this specific job) may seem much less important.
I think it might help to get your priorities clear in your own head. If your first priority is to make money for your family, then actually that frees you up a bit to look at other options, for example cutting down your outgoings (e.g. mortgage holiday) so you could afford to go part time. Or looking for another job. Or DH looking for a job. Or getting benefits (do look at Working Tax Credit and Child Tax credit like voituredepompier says).
If on the other hand you may feel strong loyalty to your employers, and this might be informing your decision - do consider that you might change your mind on this. Have a think anyway about what you might do if the baby comes and you feel like the last thing you want to do is work full time. (And you won't know for sure till the baby is here).
I know that feeling that you are needed (I used to work till 8.30 many nights with no overtime - now I think "why"?!!!), but actually no one is totally indispensable. If you suddenly disappeared would the company actually shut down? Or would they cope somehow? I bet they would. So perhaps part time might be a possibility?
On the emotional level, your DP will most likely end up a lot closer to your child than you, if you are away for a full time job. Of course this is the lot of most fathers, but personally I am too selfish - I want to have a close relationship with my baby, and I'd find it very hard if I couldn't spend time with him. You need to ask yourself if you think you will cope OK with this.
The answer might be that actually you are OK with it. I know a lovely couple where the woman is very career minded and the bloke says at home with the kids, and it works for them. Again you probably won;t know for sure till the baby comes, or has been around for a while even.
I'm not saying you shouldn't carry out your plan. But to be realistic, it is far from ideal. You have quite a bit of time till the birth, and IMO it would be well worth at least exploring other possibilities. I'm a firm believer that there is a solution to (almost) every problem, if you look hard enough and apply enough lateral thinking!