winter is not really the best time to do it in the Uk unless you are doing it an an anti/de-iced aircraft as the freezing level in the Uk is so low and the very restrictive nature of our airspace does not allow you to get above it.
Not so convinced about this for training. The bulk of the IMC rating training is done locally at the lower levels, 2000 or 3000 ft. If you're in the flatlands, while it's certainly possible for the freezing level to drop below this, it's relatively unusual.
I think autumn is probably best, as you've got most stability. Spring is almost certainly worst. I recently completed a week of fairly intense IF training in February, without weather interruption, but perhaps I was just lucky.