i know far too much about the UK speeding ticket system. Questions:
your wife has six speeding tickets on her us licence, right?
they were acquired while she was driving on her US licence in the UK during her initial one year grace period, right?
each time you sent in a photocopy of her US licence, explaining that she doesn't have a UK licence, and they sent you an offer of fixed penalty notice, you paid £60, and that was the end of it, right?
well what SHOULD have happened each time was this: your wife gets the notice of intended prosecution (NIP) stating that this car was going XX mph on this date etc. And it asks, were you the driver? She fills out the form, and attaches a photocopy of her US licence. As she does not hold a UK licence, the matter should have been automatically referred to the court. If she pleads not guilty, she doesn't need to appear in court, but can plead by post. She will still have to pay £60. However, as the UK cannot put points on a US licence these points do not go onto her US licence. IMPORTANT BIT: These points are instead put onto a UK Ghost Licence for your wife.
The problem will be that if she ever were to go for a UK licence. The DVLA will attempt to transfer the points from her UK Ghost licence onto her UK licence. And since she has 6 tickets x 3 points each = 18 points, that's a problem.
If I were you, and leaving the UK, I would call the court, tell them that you’re leaving the country, that you want to plead guilty by post, end of story. These points expire after 3 years (although can’t be physically taken off a counterpart licence until 4 years). Hopefully, if you and your wife ever move back to the UK those points would have expired.