An international driving licence basically just translates your licence into other languages. It's required for visitors by some countries, but I've never heard of anyone getting one for the UK (and needing it).
Regardless of if you have the IDL, you can only drive on your US licence for one year. After that, you'd need a provisional licence and be treated as a learner driver (only driving with an experienced driver accompanying you, no motorway, etc) until you have your UK licence.