I don't think there was a portal in this one but it sounds like what you were looking for. Buffy puts on a cream left by the trio that sends her to the beach and Spike follows soon after. He can be in the sun there. I read this on the Bloodshedverse.
Instant Sun by Verity Watson Takes place in Season Six, after All the Way, which means it is before Buffy and Spike kiss, but after she confesses to Spike that she was in heaven. Only no Giles on hand, sorry. Written because I've been dreaming of tropical islands in the midst of winter. Originally posted at BS Central. ***Winner of Judges Choice award, LLGA Round 19***