I lost my phone, well found it 5 months later it was under the ast bit of snow to melt out.

Costco for a phone, next door to Walmart for a Family Plan, well just me on it and found it worked well for me where reception is poor and am always roaming.

T-Mobile are doing an offer for a free phone ( various ok choices - can't beat free!) if you open a prepaid account and pay $100 onto the account. This is about 1.5 months worth of cell phone bills so not a huge upfront cost. I like t-mobile for new arrivals because it is really easy to set up. You don't even need a US bank account.
However, I note that metropcs are now doing some REALLY good deals now that they have been taken over by t-mobile and they prepaid. https://www.metropcs.com/metrobytmobile.html?icid=home main metrobytmobile

If you need a US phone number - then once you have arrived in the USA, you can open a gmail.com email address and get a local (to you) cell phone number through google voice. It's free to get the number and receive calls and you can download the app onto your phone.

Well I got my mobile phone sorted, Vodafone gave me an amazing 30 day trial before I left the UK, free calls and texts within the US and back to the UK for £22 a month on my UK number and contract, I took this out and cancelled it the day before it expired, I then went to T-Mobile with my now USC wife and she put me on her plan for $30 a month with free calls, texts and data. All in all it worked out OK

I have mentioned this before, but you could use togglemobile and have a US and UK number on the same SIM. The rates are fairly good, and you get free incoming calls when in the US. However you need to setup the account while still in the UK. toggle mobile I am not sure if you would be able to port a togglemobile US number to another carrier.

Why don't you just go on your wife's plan the day after you land in the US?

He already did!

I like t-mobile due to the fact they have wifi calling. This means I can now get cell service at work through wifi. I am paying $15 a month (Mint mobile) for unlimited voice and text, but I only get 2 GB of data which I don't seem to use much due to using wifi hotspots.

Need to read more. Thanks michael

What tends to be the preference here - pay a large up front sum to cancel existing UK phone contract (have 12 months left to run on mine), or keep paying smaller monthly fees on the UK contract despite being in the US and not being able to use the UK phone?

Sounds like a personal preference to me. I'd pay it off and get it over and done with. One less thing to think about going forward.

That's exactly what I done, since my relocation allowance was to wind up affairs in the UK as well as kicking things off in the US.

I was able to unlock my handset and sell it on eBay after I moved, which helped recoup much of the money (iPhones hold their resale value really well).

Does anyone have any experience with Google Fi?

Yeah my other half has had Google Fi for 2 or 3 years for $20 a month unlimited calls and texts, and just pay for what data you use, which is usually almost nothing as she just connects to wifi most places. I recently joined onto her account for $15 a month, so we probably pay $40 a month for 2 phones with the little data we use outside of work/home.

Yes the Mrs and I are both on this service. So far so good. Definitely there are cheaper options on the back of T-Mobile network (if you use a lot of data).