Ok, now this is a long shot (very long), but it could be Four Weddings and a Funeral? One of the weddings had a folky couple singing a song, and she was the female guitar-strumming hippie singer.
She had the main part in the Last Train.The train goes into a tunnel and the bomb drops.Then there was some other stuff that i cant recall.But if you go to the Spooks website they ahve tons of info on cast and crew etc.Excellent site.I think its bbc.co/ spooks or something like it.