Thanks piotrkol! Very helpful push in the right direction.TheWhiteWizard - I found a product in Poland for corn fields called Juzan 100sc with Mesotrion as the only active ingredient. Try looking around for products in the farming industry, you might find some. It's a bit of a "do it at your own risk" kind of deal but I figured I had nothing to lose. Seems to work so far!
I did a little deeper digging about mesotrione availability and indeed some products seem to be available to farmers in UK. I am hoping to take a journey today to a farm store that's a little detour on the way to the in-laws.
On a side note, and I really should have mentioned already, what a fantastic lawn you have! I read your whole journal.
Hoping that now you have a new product to help with the poa annua, you can make the progress you desire. Best regards