It is piddling down. They have a shelter so it's up to them, anyway, it's 12C, bit like having a warm shower I should think. They like the rain, it keeps the mosquitos away.
Your horses will probably be fine. Anyway, as it is 3.30am in the UK right now, you are probably (hopefully) fast asleep and they will survive a few more hours.
All ours are otu naked and have been for several weeks. Mine has just about got his summer coat through after having been fully clipped in Jan. All the rest look rather moth eaten as are losing their winter coats in dribs and drabs. You may feel it is cooler but remember they are horses, meant to live out. They do not feel the cold like we do!