dunno about the job advice, i'm afraid. you might find it better just to come clean to your new boss - say you really want the job (and you won't be entitled to SMP, but you might be allowed time off, depending on the employer), but you will understand if they prefer to give it to someone else.
But for your BFP! congratulations, and good shagging!
Oh you'll be just fine. Doesn't matter what trials and tribulations it brings, that's a wee baby in there. I like it when they come out and do that sneezing business that takes over their whole body and sends them into a state of utter shock at their own capabilities. A baby fgs. That's brilliant news.
There is nothing remotely comparable to having a tiny baby. Jobs and money and space and time and all those peripheral things just have to sort themselves out one way or another. But the baby, well that's only ever going to be an amazing thing.
Sneezes and tiny feet and chubby thighs and that one day when they wear the expression 'fuck me I'm standing up, hello world'. All of it, it's marvellous and I shan't hear otherwise.
I'm normally in 'be upfront and don't mess people about' brigade, but I must admit I would think about knocking them sidewards with my ability for a couple of months, and then say that you've just discovered etc. So long as you able to work during your pregnancy, and presumably will return to work within a reasonable period, they'd probably still rather have you than not.
And to be honest, they've messed you around in the past with their decisions.