Furthermore...

our (British) song tradition tradition is overwhelmingly an unnacompainied one

Agreed.

and by adding instrumentation, particularly with instruments from elswhere other than those common to the British Isles you are making a profound personal change to the songs.

Ultimately culture is determined by the experience of the individual. As I've said elsewhere, I've had folk guitarists get sniffy at the instruments I use and I've had vielded threats in singarounds for using an electronic shruti box. I'd say this had less to do with any allegiance to ant tradition per se as it does to the small minded pedantry one frequently encounters in a revival which seeks to enshrine the colloquial at the expense of the cosmopolitan. At tuch times it reeks of religious fundamentalism, and as with religion, doesn't bare too close a scrutiny either, political or otherwise. My Tradition is, therefore, the Indo-Euporean continuum which is born from the tribal migrations as once eloquently celebrated by A.L.Lloyd to account for the coincidences of modal melismatics from the Himalayas to the Hebrides. Hell, even MacColl was tuning into Islamic melismatics for his own vocal style, just as the celebrated Northumbrian Piper and ne'er-do-well Jimmy Allen was picking up on Indian influences on this travels!