Manohonor,
These two phenomena are quite different.
1) 'T' before 'R' becoming 'CH' as in "country" > "counchry".
This is a matter of increasing the usual strong aspiration of "t" in the "tr" combination to the point where aspirated "t" becomes "ch". Some children hear it this way as evidenced by some children's misspelling "chruck" for "truck".
A related phenomenon is the (deplorable) tendency in American English to pronounce all "str" combinations as "shtr". In the past ten years this kind of pronunciation has been on the rise, especially lately, since the American president himself uses this sort of pronunciation. (He speaks, for example, of a SHTRong America, of a good Iraq SHTRategy, of the deSHTRuction of the Twin Towers, and so on.)
2) 'T' in initial 'TU' in British English becoming 'CH'.
This is a matter of "glide absorption". The glide is the 'y' sound of the "long" U, as in "tune" pronounced "chune". This does not happen when the U is "short" as in "tunnel" -- unless the speaker really means "chunnel" (channel tunnel between France and England).
Americans make the change when the "tu" is word internal (fortune), but not when initial.