They are completely different exams.
There are essentially three sets of exams. CPL, IR and ATPL.
If you only wanted a CPL you would the relevant exams which would include knowledge specific to commercial operations.
The IR exams then contain topics more specific to instrument flying.
The ATPL exams then also include a whole load of stuff specific to airlines. A lot of this is quite generic about jet engines, or how many cabin crew you need, or fire axes, or whatever. Some of this may not be in the CPL or IR exams, as with those licences you'll never be able to fly an aircraft big enough to warrant it.
If all you want is an IR, do the IR exams. If you have any inclination of going into an airline, do the ATPL exams from the onset. It's about 5 more exams so may cost an extra £1000 or so, but if you did the IR exams first and then changed your mind you'd have to do the whole lot again for the ATPL.