There are indeed several steps between a pure glider and a powered aircraft. The criteria were historically quite simple:
Gliders with an auxiliary engine, which is just for getting them home in adverse weather, but not powerful enough for take-off. The propeller is in almost all cases retractable, so their aerodynamics is identical to their pure glider ...
I have a partial answer after more research and some help on pprune.
A Bronze + XC can be used to get a gliding licence, which under the transitional arrangements can be used to obtain a LAPL(S).
In theory the route from there to a LAPL(S)+TMG and on to a LAPL(A) TMG is feasible, however the BGA is not an ATO yet and there are very few instructors or ...
The easiest way to find out for a particular aircraft type is to check its (TCDS) Type Certificate Data Sheet. In it, you will find its certification basis and airworthiness category (CS-22, CS-23, CS-25, CS-VLA etc).
EASA TCDS's are available for free at:
FAA TCDS's are also available for free at: