You can't apply any more for a UK CAA ATPL.
It has to be a JAR ATPL and you will loose your IMC rights when you do so.
Its quite striaght forward.
After you have the required hours.
Then sit a LPC and pass it.
Fill out the form and try and invent numbers for some of the requirments because you have never logged Instrument time etc.
Pay some money
Send off your log books and form and 3 weeks later a green book turns up.