I did the relatively simple excercise of upgrading a NPPL SSEA to a JAR PPL. Relatively easy as the exams are the same. But the paperwork excercise was a PITA. Plus re-doing your Qualifying Cross Country flight is an expensive excercise (tip - whichever route you take make sure your QXC is 150nm)
My advice for what it's worth is if you have any aspiration whatsoever to upgrade to a 'full' i.e. JAR/EASA PPL then simply do that from the word go. You then do the 7 ground exams once and they're done. You can then fly TMGs, microlights with straightforward differences training.
I called in to Hinton recently. Great place, very friendly. If cost is paramount then the TMG route to PPL can save money. In the real world, you need to think where you are actually going to fly from, i.e. what is available near to you? If it's a great TMG place like Hinton then the TMG route makes sense. Likewise, if it's a club airfield where you can rent a decent little C150 at good rates then clearly the PPL route may make more sense.