I don't think I'm as ill-informed as you think I am, but okay, if you think you can save me some money, that's all good.
Oh yeah, I'm gonna knock your socks off.
But certainly tell me if something else seems cheaper. Zip code for the summer house is 47246.
T-Mo coverage. Excellent. Let's lay it out for you.
Using Google Fi for your phone service means you're committing to a $200 phone and $20/month phone service for "unlimited" talk and text. For what you're using and needing? It's absurd.
Google Fi solution:
$200 Nexus 5X (be sure to get the international version and not the US)
$260+tax for 13 months of phone service
$60 data
TOTAL - $520+taxes
My first solution:
$30 T-Mobile hotspot (ZTE Z64 MiFi - resellable)
$35 T-Mobile prepaid 6GB data
(potentially cheaper or possibly a wash if you just ride out one more month of mobile data with AT&T
out of contract or switching to prepaid - do the math)
$100 Alcatel Pop 3(5) Android handset
$20 Google Voice number port
$10 one month Airvoice, H2O Wireless or Puretalk USA phone service (plus $1 for SIM through dealer)
(probably cheaper still if you stay with your current carrier until you port your number to Google Voice before you leave - do the math)
$90 GiffGaff 125min/500SMS/100MB plan for 12 months PAYGO
(or) $72 TalkTalk 250min/unl-SMS/300MB plan for 12 month contract
TOTAL - $286+taxes (or cheaper)
Nearly half the price.
For mobile data at your cabin for work, straight up T-Mobile prepaid data plan with hotspot if you can't get AT&T prepaid data rolling on your existing device.
If you want to go 100% no contract and roughly PAYGO for usage for phone service domestically and abroad and your usage is going to stay pretty consistent, one recommendation would be H2O Wireless Stateside given the minimal usage (with Google Voice as your primary number - just remember to disassociate your "hidden" US number from the GV account when you let it lapse) and GiffGaff for your UK usage and just swap SIM cards. Switching to an Android smartphone, however, will make it easier to use Google Voice/Hangouts for texting and phone calls both home and away.
Alternately, you could go Truphone SIM for your US/UK service and just keep using a cheap feature phone so long as it had GSM/UMTS 850/900/1900/2100 band support. They offer free incoming calls and texts both in the US and UK, so there's the potential for greater savings, but worst case with the numbers you stated? $6.75/month for US usage, $10.80/month for UK usage, you could port your US number to Truphone, and it would only be $8/month for a UK number. All calls to both numbers would come in on your phone with zero hassle.
My second solution:
$30 T-Mobile hotspot (ZTE Z64 MiFi - resellable)
$35 T-Mobile prepaid 6GB data
(again, potentially cheaper or possibly a wash if you just ride out one more month of mobile data with AT&T
out of contract or switching to prepaid - do the math)
$50 Nokia 6350 (used unlocked - only if your current phone can't be used)
$136.35 Truphone (at most - probably much cheaper)
$96 UK phone number for a year (if needed)
(or) $27 UK inbound phone number through Localphone (no SMS) for a year
TOTAL - $347.35+taxes (or cheaper)
The thing is, you don't go into detail on what you currently had. It's only been with prodding that you even mentioned you're with AT&T on your mobile data. Depending on what phone you currently have, you could easily optimize and knock your prices out of the park while still getting everything you need and more (and more simplified for doing the US/UK divide) than the way you're probably approaching it currently, for roughly half of what you're committing to if you go Google Fi, and it can be just as convenient.
Republic Wireless
There is always a cheaper and better solution. The $5 RW plan isn't anything more than you paying a price premium for using what is effectively Google Voice, which is a free service, only you're locked to a proprietary handset.