Is it too late to plant some garlic?

The thing about garlic needing a cool spell to make it split is true. However I believe the requirements are only a soil temperature below 10 degrees C for 3 weeks so you should have no worry in the UK. If you live in southern europe or the Canaries it might become an issue

They grow wonderful garlic all around the med, with very little frost.............but it's always softneck garlic. Perhaps their varieties have been selected so that a cold spell is not required