Its definatly something i'm thinking about, but as you say "train in the uk to work in the uk" You would still need the FAA multi/IR/CPL before you could do the FAA flight instructor ratings and i think the cost would be $35-40k. plus the JAA Conversions. EFT were qouting $63k plus expenses etc for the PPL right upto the ratings and an instructing position afterwards.
The only real advantage i can see is that you would be building instructing time for CPL and instrument. However this would only be of benefit if the CAA recognised this time and took it into account against the requirements for IR instructing (200hrs instrument/800 IFR).
I realise that theres no shortcuts and it would probably just as difficult as going doen the JAA route. I imagine that you would start as a FI once back in the UK. But i could also see that it would be very difficult to build the instrument time needed for IRI unless you went down the AOC route for a few years (no bad thing).
if anyone has comments i'd be greatful to hear them