Who are you currently flying for??
Most of the time you can get an ATPL issued whilst doing your annual LST in the simulator or aircraft. I assume that since you need the ATPL to take a command you must be working for someone who operates multicrew aircraft, why not just get them to sign the ATPL flight test form during one of your checks??
Otherwise it might be just as cheap to hire an aircraft or simulator for your current type and just book a CAA examiner. Getting a foreign licence and then converting seems a long way to go about it.