Andy

A forgery prosecution in England and Wales would be predicated on evidence of an attempt to gain a form of benefit under the laws of England and Wales. While technically you have committed forgery in the context of signing your wife's name on US immigration documents, the benefit you sought (accidentally or otherwise) is not within the jurisdiction of English courts, so there is no realistic hope of conviction. As such a prosecution would be unlikely, unless the US embassy forwarded details to UK authorities and requested a prosecution. Unless an argument could be made against you that you were seeking to avoid something such as payment of a debt legally owed in the UK, by emigrating before action could be taken against you, there would be little of a viable case for a court to hear.
 
If you are a resident of Scotland, the law is a little different, but even then, without the US embassy forwarding details of the fraud, the authorities would have no way to know it had been perpetrated. 
 
The benefit you sought was under US law, and you could face criminal charges there, under federal law (US immigration is federal, so the benefit you sought was in federal jurisdiction), but unless you are within the jurisdiction of a federal court (meaning you are present within the US or one of its territories), the authorities there cannot act against you directly, other than to bar you from entry. They could, of course, seek your extradition, but that seems very unlikely when they would see a bar on entry as being significant punishment.
 
In any event, seeking to withdraw your visa application may help mitigate any further action than a probable ban.

Business days are Monday-Friday, but there are work process programs and volume targets to meet that mean that yes, some USCIS staffers could well be working over weekends. For the last few years this has always been the case, so there is no reason to believe that the presence of a 'touch' on a Saturday means anything other than that a staffer logged an action relating to that file on that day.