It depends on which Republicans. And whether Germany invades Spain.
If they are reasonably level-headed socialists, the UK holds its nose and ignores the odd church-burning or nobility-lynching and makes nice. There's a lot more dangerous and odious countries out there, they want to keep Gibralter protected, it might be nice to have the option of a front in Southern France at some point, and they'd rather the Stalinists don't get in. (IOW, a bit friendlier than OTL Allied policy to Franco's Spain, since Republican Spain is presumably not providing aid to the Axis.)
If it's Stalinists, then Spain is treated as part of the USSR as far as diplomacy goes.
If Germany invades Spain, do whatever it takes to hold Gibraltar, plus there's a lot more coastline to raid and partisans to supply, but on the downside there's even more U-boat bases.