Rajoy says the referendum has to be country-wide.
I think the British Tories are much much less passionate about unionism than the Spanish Populares, partly because the Populares know that the Catalan issue helps them stay relevant whereas the Tories have no stake in Scotland.
However, I think May is one of the few who is passionate about the Union, and Sturgeon is exploiting that to bang the nationalist war drum. Ganesh of the FT was on to something when he said this was a move designed to strengthen the SNP's support after a rough time.