I've just done a bit of digging in Lasors 2010 and this is how I see it.
You can obtain a PPL(A) in a TMG and add SEP as a type rating.
To go for commercial you need (from D1.2) hold a PPL(A) (it does not state that the PPL must be fully SEP, only that it is a valid PPL) and have 100 hours PIC.
Of those 100 hours PIC 30 can be TMG. Since during training you are not PIC unless solo (does it count as you are still under instruction? I need clarification) you could potentially do a further 30 hours in TMGs whilst you are hour building.
The relevant bit is the final 200 hours gained up to CPL level, of which it states the 200 hours can be a combination of Dual and PIC of which 30 hours TMG PIC are accepted.
So, it seems to me the TMG route is valid and therefore very cost effective.
As with LAI, ready to be corrected.
LASORS 2010 - Relevant sections are C1.2 and D1.2(D)
LASORS 2010 | Personnel Licensing | Safety Regulation
This is my planned route to a CPL so am very interested in the outcome of this thread.