yes they can.
if your trading bitcoin for fiat. then the country that owns/licences/tenders the fiat can intervene on anything involving their fiat.
even if 50% of the transaction is bitcoin related. if fiat is involved, the bitcoin part of the trade wont absolve you of any fiat responsibility.
however.. as an idealist, here is my opinion
if bitcoin was traded for goods/services then its no longer treated as currency exchange/money business, but acting as a retailer.
so if we take away the bitcoin<->$£ valuation and instead replace it for a btc<->'COL'(cost of living index).. or to put it simply 1btc=10,000 loaves of bread.. then bitcoin can be free from the fiat regs and government wrist slapping
Yes, govt can manage fiat money in their own way to undermine bitcoin. And if bitcoin would be a real treat to fiat money hegemony (exchanging goods and services for bitcoin), then i think govt of every single country would be searching, for possibilities to regulate bitcoin ecosystem, and turn btc to exclusively govt owned system of wealth transfer. What you think about this possibility?