With the dollar slumping to a 26-year low against the pound, already-expensive London has become quite unaffordable.
主干：...London has become unaffordable.
这是个简单句，句首的with+宾语(the dollar)+宾补(slumping to...)作状语，表示原因。
译文：随着美元兑换英镑跌至26年来的最低，原本已经很昂贵的伦敦变得让人去不起了。