Do a quick search on here for 'Dragon Helicopters' then another for 'Bristol Ground School'. The results will help you make up your mind!
Otherwise, this topic (ATPL or CPL groundschool) has come up many times before. Another search will produce the thread. Essentially, if you are going to go to the effort of sitting the exams, you might as well do the full ATPL syllabus.
And Dragon are not the only CPL providers. Off the top of my head, Atlantic Flight Training @ Coventry and GTS @ Bournemouth also do it.
HTH,
B73