Gcse help, confused, worried

I have recently received my results back from my GCSE English retake and unfortunately achieved a D. I want to join the Royal Engineers and cannot do so without a C in English, but i have now left college so i was wondering what i should do next?

How would i go about retaking it again now that i have left college and would it cost me and who would i need to contact in order to achieve this?

I'm mot an over achiever and simply need a C in English Language, so i was wondering can you simply just do a Foundation paper or a IGCSE paper to achieve this or would it need to be Higher GCSE???