Quite right you require the CPL ground exams, currently only the National ones are available, but as soon as there is a CPL theoretical course up and running I hear the exams will be available.
You do not need a CPL to instruct, only a PPL ( and Class 2 medical) but you cannot be remunerated. And of course after 36 months the exams you have spent all that effort on are no longer valid to complete the CPL.
As Beagle says the NIPPLL is on the way, the Nationally Invented PPL, lower hours, lower standards all round but I am told you will still have to have CPL level knowledge to instruct!!! I am lead to believe that this has not yet got beyond the proposal stage and might take a long time coming if it ever does.