Lock Android device to single roaming partner
 

I am a T-Mobile USA customer. When I visit (for example) the UK my phone and tablet can roam onto a few networks:

EE (sometimes still displayed as T-Mobile)
O2
Three
Vodaphone

..problem is that if the device loses its network connection (as when in the Underground, but could be anywhere) it may reconnect to a different network. There is often a several minute delay after the new GSM connection is made until the data connection is usable, during which interval Hangouts and Maps etc are unusable.

Have I overlooked a way to force the phone to connect to one, and only one, roaming partner?

You should be able to manually select the network you want.

I can to start, but if it loses the connection the devices will roam onto any available network.

There's generally an "automatic" or "manual" option. Ensure it's set to Manual and select a network. For example, on the iPhone you can either choose the carrier automatically or select one manually. If you do the latter, it'll always look for that one until you tell it to pick another.

If your phone winds up picking another network when you regain service, then it's either something to do with the specific phone model or it's T-mobile's SIM not letting you lock on to one specific carrier.