Sudden braking from a fast speed might be more likely to cause a skid (same as a car but scaled down), but if you're popping down to the shops or commuting to work, you likely will be cycling in a more relaxed style anyway. If you're uncomfertable with braking in the rain, I recommend finding a quiet strip on a rainy day and seeing what it's like. Actively try to skid so that you know what your bike's limits are - from decade of experience cycle commuting in a famously rainy city with some highly aggressive drivers, I've never found the rain to cause additional issues beyond getting wet if I'm unprepared. The only time I have ever come off my bike by my own fault was skidding over a patch of ice about 9 years ago, so I would recommend higher levels of caution in the snow.