I've set up Google Voice -> US SIP -> UK SIP -> 3 Mobile. This gives me a US number to give out to Americans (who don't want to dial my UK number) that they can text/call. I can then make calls when on Wifi or using a Phonecard app which automatically dials my UK SIP and forwards the call via the US SIP provider. I don't have to switch SIMs or give up my UK number.
This works well for me (since I travel between the US/UK often)
Can't you just use Google Voice on Three's 4g connection/Wifi to make/receive calls/texts on your Google Voice number using Hangouts?
What's the purpose of the two SIPs?
Three doesn't have any 4G when roaming, and the 4G isn't that great even in London (compared to say: Vodafone).
Texting with Google Voice is fine though.
The two sips are registered to the same installation of asterisk: The UK provider gives me a UK number and rates equivalent to a UK landline.
The US provider gives me a US number (which is registered with google voice) and rates equivalent to a US landline.
Sending a text from the UK to a (for example) Spanish number incurs a cost outside of your allowance. So that same text would also incur a cost when you are in Spain.