Will the SIM card work BOTH in the UK and in the US?

If you have a US or global SIM from us and an unlocked handset you can set it up in the US (remember to enter the PIN pre-fixed with a 3 in the US) and test it - make sure you read the manual fully. Then, inthe UK, providing your handset is suitable (check with your US network) enter just the PIN and it should be fine. If not call us or email us for support.

Thanks so much for your response.  I feel safe to check it now.

I put the SIM card in and apparently I have service--I attempted to make a phone call just to check and received a text from ekit saying to stop and put in the UK pin code for cheaper calls.  I did not receive a text with a US number in it, however.  I don't NEED a US number, but I wondered if that means there was some problem with the setup--or since it powered up and I was able to receive a text from ekit, does that tell me I'm fine, the phone is unlocked, and I'll be ready to go once I reach the UK?  Thanks again.

You should be fine. Your US number is available on the top up web site. Please just ensure you enter the PIJ correctly for the country you are in - see below;