I read a book series in 2009, can not remember the title of the book nor who wrote it.
I remember it started with two males driving at night through the country. they pull over and walk into a paddock, spot a cave, and walk into it, (from here I'm unsure if both disappear or just one of them). Anyway, one male comes to in a different cave, walks out over a hill where he looks onto a battle between two armies but they are fighting with bows and spears and swords, with paint on their bodies and flying half ripped flags. After the battle finishes, someone 5-10 years older walks up to him and says his name. Turns out it's his friend who walked into the paddock and cave with him moments before, but he has already lived 5-10 years in this time.
Their present is based around London 1980-1990 I think. The past is based in a Celtic time before gunpowder, but around the start of stone masonry.