Should we perform only in our own tradition to preserve it?

Which "we" do you have in mind? There are a lot of different people using it, with their own reasons for folk music to serve many different purposes.


If so, at what point in time to we lay down what the tradition is? Do we perform only English traditions as they were in the 18th and 19th centuries?

That we can be fairly definite about. Both tunes and songs from that period arose in a well-connected international culture. Any time you play a tune with I-IV-V-I chords you're using an Italian idea from the 16th century, and it was obvious by Child's time that most of the best-known ballad stories had cognates across Europe. The material English folkies perform today was never part of a purely English tradition, and you can say something analogous about every other part of the British Isles. Scottish Highland bagpipes were introduced from England in the late 1400s, were always made outside the Highlands using African woods, and for the last 150 years have usually been accompanied by drumming that comes partly from the continental European and Ottoman military traditions and partly from colonial Africa.

The music most people heard most often in the 18th and 19th centuries in the UK was hymns and psalms. Which mostly originated as a collaborative effort by Calvinists from all over Western Europe and never developed any regionalized idioms - the only ones that look that way (like the psalms of the Western Isles) are relics of once-widespread repertoires. The instrumental music that formed the core of most practical music making at the same time was in the army, and you played much the same stuff in the same way in military bands in Peterborough or Petersburg.

Cherry-picking the historical record to create a "national" idiom is inventing a past that never was. It may sell downloads and put bums on seats, though - which gets back to the your first sentence - who is this "we"?