Hello mate,
There is no answer to your question. From the companys I have worked for Line Maintenance covered all scheduled maintenace upto A checks and any unscheduled rectification. This means you could carry out a scheduled or unscheduled engine change.
However you need to forget how you worked in the RAF and read EASA Part 145 and EASA Part M. These documents are not light reading but they lay down the requirements to be met. You then need to submit an exposition to the CAA detailing how you will comply with the Part M and Part 145 regulations. Stuff like key post holders, maintenace facilities etc etc. If you try and do it all from scratch you will tie yourself up in knots.
My advice would be identify a company that is of a simular size to yours that is already approved to carry out the type of maintenance you are trying to do and consult them.