If you want to eventually work for an airline, do the ATPL exams. If you simply want to be an instructor, do the CPLs. Nick is right, there aren't many places who do the CPL course. CATS is one, and I believe Bournemouth also do it(?).
There are 14 ATPL exams, versus 9 CPL exams, so you've saved £400 in exam fees straight away. Things like global climatology, transatlantic and jet flight planning etc are a complete waste of time and effort if you simply want to instruct. It's been said that the CPL exams are about 60% of the work required for the ATPLs, and the IR exams around 40%. I've recently seen some guys blast the IR exams out in 3 months.
Also worth noting that there is only 1 place you can sit the CPL or IR exams; Gatwick. Plus they're done on alternate months; CPL one month, IR the next. Fail an exam and it's a 2 month wait for the resit (although that's probably the same with the ATPLs anyway since they take about 3 centuries to mark the feckin things and send your results out).
If you ever wanted to do an IR, that's another 7 exams with some of them being duplicated to a large extent (eg. Met, Air Law), whereas some are considerably different (eg. Nav, Flight Planning). The IR Human Performance exam credits for the CPL, but not the other way round; so potentially you could save yourself sitting it twice by just doing the IR version.