If you dont stay current or have a proficiency check you will need to retake the cpl skills test. Cost maybe £3000+
I am not sure what you mean by this, but I do not think this is the case. Once you have you CPL it is valid for 5 years, and getting it renewed is a paper work and parting of cash to the CAA exercise. The rating you need to keep current is your SEP, as you mentioned.
To answer the second part of your question P1F, even though you have shelled out the cash for your FIC, you will find that you wont need any practice to pass the necessary tests, as your flying skills will be enhanced and you will be current.