As far as I'm informed, you do not need (CAA wise) a UK license to fly a UK registered aircraft. Your (UK) company may require you to obtain a UK license, but that is only so their trainers can sign your license when you do your sim-checks (LPC/OPC). Else you have to send all the OPC/LPC paperwork to your local CAA (state of issue!) and then they must sign and stamp and whatever they do. Having a license issued by the state where the aircraft is registered just makes everything easier paperwise.
With regards to ATPL; in a lot of the JAR countries, you don't get an ATPL license unless you have a command check (left seat!) on a multi pilot airplane. In the UK, you can apply for an ATPL when you have 1500 hours total time (+some nighttime and crosscountry etc.).
If you come from outside the UK with a non-UK JAR CPL, and have more than 1500 hours, you should contact CAA in Gatwick in order to convert your state of issue to the UK, and at the same time apply for an ATPL license. It will cost you around £220 for the license and £22 for the medical conversion.
Happy landings.
