What is another emergency number used in the European union does it work in the UK?

The number you are referring to is: 112

This was introduced as a common Europe-wide emergency number (not just the EU) to replace all the different numbers in different countries, so people would know what to dial in an Emergency wherever they were.

112 does work in the UK, and it is the same as dialling 999. But it has had very little publicity in the UK.