My fiance from England is visiting, if we marry can she stay in the US or does she have to go back to the UK and await a Visa? I am divorced father of 3 kids and it would be very stressful for her to have to return to England.

if you get married now in the US, it is likely that the CIS assumes visa-fraud, meaning that she entered on a tourist-visa with the intent to marry. this assumption would make her subject to deportation.
so unless you are certain that you can convince CIS that this wasn't the case (which will be very hard if you were already engaged when she came over) I suggest that she goes back to the UK and you apply for a K-1 (fiancee-visa)for her. with that she can come back legally, you can get married and start the AOS process. the K-1 usually doesn't take that long either

just wanted to tell you that K1 does take lots of time. i came here on fiance visa. we filed in end of december at INS office and i had my interview in May.

so please keep that in mind..it is really very stressful process.
wishing u all happiness of life

I don't want to give bad advice but I was told by 3 separate lawyers that if someone came over to the US on a 90 day visa waiver and got married and filed for the AOS just before the visa expired or even after, then the issue of "intent to immigrate" wouldn't come into play. When my wife spoke to the USCIS, she was told that it was quite legitimate for someone to come over on a 90 day visa, meet the love of their lives and get married all within the time span of the I-94.
However, the above advice is worthless if the USCIS knows that you are engaged Ector. If they don't then it might be worth your time to call round a few lawyers just to get their opinion.

The answer depends alot on some details you do not provide.

If you married within 30 days of her arrival, it will be very difficult to overcome their presumption that she intended to marry when she entered, and if you lose she will never get her reseidency.

If you married after than 30 days, then it is not automatically a problem, but it could be depending on how she answers their questions about her intent when she entered. As Paddy said, if you were actually engaged before she arrived, that would be a very bad piece of evidence against her.