Reliable SIM Card for England

https://www.google.com/amp/s/www.techadvisor.co.uk/test-centre/broadband/best-phone-network-2018-uk-3452450/%3famp

Bit of bedtime reading there.

EE should have the widest coverage but if you are anywhere reasonably populated the big 4 networks will be available.
If you are in the mountains of Wales,northern England and Scotland you can not be sure of signal.
Whichever operator you choose will be much much cheaper than your US carrier.
I am alongstanding giffgaff user although on price carphone warehouses own brand SIM is great on price,it runs on 3's network

If your first night is in London odds are good that several competing phone shops will be in walking distance. You can identify their locations online before your departure by mapping the location of your hotel/B&B and then searching for "phones" or "cellphones" or "mobile phones".

At least one of Heathrow's terminals has a shop selling SIM cards. (Not sure about Gatwick.) Cards are also sold in vending machines but I'd rather deal with a person who explains plan options/prices and gets the phone working for me.

I’ve been with O2 for 10 years and they are all round the best network imo, if not the cheapest... but i struggle to get 4G in some parts of Zone 1 London... forcing the phone to 3G gets my net working again, if a little slower.

I purchased a Vodafone SIM and a big value bundle for my London and Switzerland trip last year. £20 and it worked fine for my purposes. Planning to do the same for this year’s holiday.

Delayed but huge thanks again to you all. Just ordered a sim from myUKsimcard.com based in Idaho, in case I don't have time once I land there. But if I do, every terminal in Heathrow has a SIM store. Fingers crossed, this works!