Since my smartphone does not offer an international plan, I had intended to buy a SIM card once I landed in the UK. Since then I read about Google Voice on one of these forums. My question is: if I use Google Voice to call or text members of my travel party who all have US numbers, is there a charge?
I don't believe so. I usually don't bother since I use Audio FaceTime to call family and friends (they all have iPhones), but it is possible.
Here is the tutorial from Google Voice that gives you some more explicit information: https://support.google.com/voice/answer/7491761?hl=en
Pretty certain Google Voice would work fine for calling/texting US numbers from overseas, but note that Google Voice requires data. So since your current plan is not providing data, you would need to be connected to wifi to call or text them. That would be a problem if you were all wandering about away from the hotel. IF your smartphone is unlocked, you could buy a sim card pretty cheaply, which would give you data. But note that if you replace your current sim with another, you will no longer have your "normal" number, until you switch back.
Actually with newer dual SIM phones you can have access to texts and calls on your US number. It all depends on your US phone plan and how you set up your phone. My US plan allows wifi calling and texts. I turn off data roaming on the US service. I load a UK eSIm plan that gives me data and a UK phone number for local calls and texts. UK has super cheap plans. For around £10 I can get 30 days of service and plenty of data.