Hello.
You won't go to prison.
I had a similarly high reading at the roadside, confirmed by the subsequent test at the station. I'd also managed a low speed collision with a (thankfully) empty parked car. Like you, I too was aware that a custodial sentence was a possibility and it completely freaked me out.
I got a 33 month ban (reduced by 33 weeks if I did the driving course, which I did), an HRO designation,150 hours community service and two years supervision by what used to be called the Probation Service, which meant a weekly, then subsequently a monthly meet/phone call with my case officer. I did the community service at a charity shop.
As I left the court and digested the sentence, never has the exhaust-filled air tasted so sweet
Though the sentence was awkward and a hassle and tricky at times, I nevertheless - and this is very weird - look back at dealing with it almost with nostalgia. I met some extremely interesting and nice people on the journey. I was healthier and had more money.
Reading other comments here, it seems that you may well get a more sympathetic response from your employer than you might have imagined. Get a solicitor (they're all much of a muchness, I reckon), communcate the genuine remorse you've expressed here, do the stuff the courts give you to do and move on. Don't drink and drive.
You're not a bad person. Thankfully no-one was injured or worse. Life will go on and you will get through to the other side of what I know is an extremely unpleasant and stressful experience
Good luck.
P.S. I still work as a volunteer at the charity shop.