A notarized translation for the US is a certified translation with one extra layer: a US notary public notarizes the translator's signed certification of accuracy. Some US universities, employers, DMVs, courts and state agencies ask for exactly that, and it tends to arrive as a single line in an email that leaves you wondering where on earth to find an American notary.
You don't need to find one. We translate the document, certify it, have it notarized by a US notary public and email you the notarized PDF. You order it online like any other certified translation, from the UK or anywhere else. And if your document is going to USCIS rather than a school or employer, read the next section first: you probably don't need notarization at all.