After What's My Line Spike and Buffy leave together in mutual grief (I think Angel and Dru were dusted by the order of T/kendra or some such).

I think you are looking fore the Alone Series by SpikesKa

Alone With You
Summary: Set during What's My Line and going A/U from there.

Alone No More
Summary: The sequel to Alone With You. Spike and Buffy are together in England, working side-by-side to take out the Order of Taraka.

Never Alone Again
Summary: Third in the "Alone" series. Picks up where Alone No More left off. It's time for the showdown with the Order of Taraka.

*SpikesKat