1. Why should I register a UK trademark and not a CTM?
When you register a UK trademark you may face oppositions from holders of UK trademark owners and CTM owners. When you register a CTM, which will be valid in all 28 EU countries, you may face oppositions from holders of CTM owners, from holders of UK trademarks as well as holders of national trademarks from any other 27 EU countries.
2. When would it be better to register a CTM rather than national trademarks?
When you plan to use the trademark in the UK and other 1 or 2 EU countries, it is advisable to seek registration of a CTM. That would be cheaper and, possibly, faster. However, when you register a CTM you may face oppositions from holders of trademarks from any of the 28 EU countries.(For example, a trademark owner from Latvia may oppose the registration of a similar CTM, but may not oppose a UK national trademark application).