Complex sentence: In doing so, he annexed what is now the county of Cumbria to England; this final annexation established what would become the traditional borders of England, which have remained largely unchanged since then (except for occasional and temporary changes).
Split into simpler sentences: In doing so, he annexed what is now the county of Cumbria to England. This final cession established what would become the traditional borders of England which have remained largely unchanged since then (except for occasional and temporary changes).