The problem you have is that FI training has to be carried out through a FTO with approval for FI courses. Ususally they will wish you to use the school's aeroplanes, but if the aircraft is properly maintained and insured it will usually be possible to get exemption from the CAA to use your own aeroplane. However it may be that you will have to take your aeroplane to a school which has approval and base it at that airfield while you do the course, since there is such a large ground school element to the course (125 hours). If you are planning to do the course full-time, then you should allow about 5 or 6 weeks at this time of year. To find a school then check the CAA website for FTOs.