There is effectively no conversion to a UK (JAR) CPL. You would have to start over from scratch. Any experience gained outside the UK and any foreign licences wont count since UK standards are so much higher than the rest of the world. (as if!)
You will be required to complete :
26 weeks ground school
14 written tests
2 flight tests
1 medical (just bend over and try to relax)
You will burn up lots of money (£1 sterling = AUD$3) and a Seneca II with instructor will set you back about £350 per hour (that's AUD$1,000 per hour) plus landing fees and instrument approach fees.
Your ground and flight instructors will work hard to assure you that you dont really know how to fly and you will start to wonder yourself since all the UK rules and procedures are designed for a five-MAN (oh god you're not female are you??) flight crew....that's 2 pilots, 1 engineer, 1 radio operator, and 1 navigator...and the instrument rating test flight is usually done single pilot.
The only good part is that you dont have to work very hard or very smart to get ahead in the UK.
Best of luck.