You don't convert an FAA CPL to an be ATP, you meet the requirements and get issued it. Other than hours, you'll need an ATP-CTP course, then the ATP written, then Oral and flight test (assuming you want AMEL). Doubtful you'll be able to do the ATP-CTP (a bit like an MCC) in the UK. You might find an examiner for the check ride. If I remember correctly you doing need a CFI sign off unless you fail!