2 weeks in London, then back to USA, which Tmobile prepaid phone?

I am going to be in London first two weeks of August, and I want a
phone I can pop a sim into when I arrive and use during my stay, then
pop in another sim when I get home (USA) and use it long term to
replace my old verizon cell. We already have a Tmobile prepaid,
apparently the only Nokia that won't work in the UK, but we really like
Tmobile and want to get a phone that works with them.

I have found a few good general reference sites, but I am running out
of time and need to buy a phone and sim card ASAP or wait and shop once
I get to the UK, but that feels extra trip risky messy.

Basic phone is all I need, text and voice, so cheap and reliable and
not too hard to find. Just a model number to work with would be great,
but any links to specific places to get the sim cards much appreciated.

Unlocked triband searches haven't narrowed the issue down enough.

.

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