Texting between the UK and the US is always hit and miss - at least that's been my experience over many years using all sorts of combinations of networks, with both US numbers and UK subscribers roaming in the US.

Sometimes it doesn't work at all. Sometimes it works in one direction but not the other.

As others have mentioned US cellphones often come with email addresses (the usual format being some variation of "number@network"). I can text American friends by sending email to their phone numbers (and they can reply, and the reply arrives by email), but we can't reliably communicate phone-to-phone.