The PMR446 standard restricts transmission power to 0.5W and the aerial is fixed and short. I don't think you're supposed to be able to take it off with a 'twist and lock' system and replace the aerial with something better. The idea (I believe) is to restrict range so more people can use this frequency without interefering with each other.
If you do a search on Google, you'll find some websites show how you can put a coil around the fixed aerial and attach that to a larger, external jobbie that apparently improves reception and range.
Cheers, Norman