As a newly qualified UK (CAA/JAA) pilot, I'm wondering what the
difference is between:

CPL/IR (Commercial Pilot Licence with Instrument Rating) - often
called a "frozen ATPL"
ATPL (Airline Transport Pilots License)

For instance, the medical requirements are both Class 1, they both
enable you to be paid for flying (including Air Taxi) and they both
allow you to fly jets (with the appropriate type and turbine ratings)
so what are the differences?

Preferably, in addition to the question being answered, an URL
pointing to the relevant CAP I could further research this would be
great accepted.