I was going to add TMG to my licence and was told at non-UK CAA that it could be added to either SPL or PPL, but not both same time.
I assume it's to do with the EASA requirement of only ONE EASA licence with the same class/category privileges. The fact that some people may have Spanish or Greek PPL/CPL and then get British or Irish etc, without surrendering the earlier, is another issue.
The EASA regs have slightly different training requirements for adding TMG to LAPL, SPL or PPL, obviously due to different cross country etc requirements for licence issue.
There is an advantage to keeping TMG on PPL - revalidation by experience (plus the flight with instructor) on PPL can be done through both SEP and TMG - their combination, rather than SEP only. Just something to consider, wanting TMG on SPL.