Yes, +44 is the country code for the UK. The next four numbers are like an area code or exchange. The last six are the number. So assuming your WhatsApp is connected to your US number then you should add the contacts with +44.
If you end up getting a UK number, for example if you pick up a prepaid SIM card, then it’s a little different to make a call. You drop the +44, add a 0 (zero) to the front then dial the rest of the numbers. The +44 should still work for creating the contact for WhatsApp in this case.
For example, either:
+44 1234 567 890
OR
01234 567 890