Mate,
Give us a chance, you've waited a whole 2 1/2 hours on a Sunday afternoon before becoming impatient for a response!
I am in the middle of converting an FAA CPL IR Multi into a JAA/CAA license.
First things first you have to do the 14 ATPL exams. There are a couple of schools to choose from but Bristol and the Oxford seem to be the front runners. This will take 6 months if your rapid more likely a year plus for home study whilst working. Don't forget the four weeks mandatory brush up in a class room before your allowed to take each tranche of exams.
With the Exams passed you then need a minimum of 15 hours IR training of which 10 can be in a sim. This is a minimum and it is likely to take longer as you will be tested on an NDB approach which we don't have in America.
For the commercial you can take the check ride immediately but it would be arrogant to assume you'd pass without training into the UK nuances required. It seems the average is 10 hours.
Budget 10,000 Pounds for the flight portion and that excludes Multi Crew Training which isn't mandated but has now just become the done thing to have.