My cats have never worn a collar either. They are all indoor/enclosed outside access.

If you want your cat to wear a collar, then as has already been suggested the breakaway ones are probably the safest, although I would check it first to make sure that it does actually come apart. This site Kitty Collars: Safe Cat Collars UK - Cat Safety Collars UK looks quite good, the ones with messages on look quite useful. The only thing with microchipping is its not visible. I do wonder how many people take in what they think is a stray but never bother to find out for sure by taking it to the vet to be scanned for a chip.