Andrews and Arnold do an interesting service where they supply a SIM card which connects to VOIP at their end. Annoyingly, they don't have a sensibly usable set of 07... UK mobile numbers they can route onto VOIP to go with the service, but since my OnePlus Two has two SIM slots, that seemed like a way to give it a punt...
The particular variant of SIM I ordered (O2/EU Voice) doesn't push out to Nano SIM, instead requiring a pair of scissors and a steady hand (or a proper tool, but who has one of those?) As you can see from the picture, I got away with the scissors and it even worked afterwards:
Android has some pretty impressive native support for more than one SIM, and shows two signal strength selectors as you'd expect. As you can see at the top left, the SIM networks (operator names) are shown with a pipe separating them. For some reason you can't fiddle with this on the control pages, but you can set it when ordering and contact support to alter it.
I ordered a London number on AAISP's VOIP service to go with the SIM, and that all works as expected. Texts are a bit clunky (presenting as from "SIP2SIM"), but it looks like that may be configurable/changable.
Mobile data appears to go via NAT and emerge from an IP address registered to Manx Telecom.
The two things I really wanted to play with are setting up my own Asterisk again, and using the roaming to get decent data on the train up north. I'll report back when I've had a play.