Yes its odd, TMG is a JAA rating, SLMG is a national rating - a relic of the old CAA administered regime, pre JAA. There isn’t much difference in terms of how the two aircraft classes are defined, as far as I'm aware.
you can still keep your SLMG rating when you convert your national UK licence to a JAA PPL, or upgrade to a CPL/ATPL.
dont know what will happen when EASA takes control...