No UC experience (as in infil), but I often work plainclothes.
My fav holster for that,personal opinion, is a kydex inside-the-pants holster from Front Line. Only holster security is a tension screw. Only drawback is that it gets painfull to wear after a while (I am a skinny dude
)
I find it a great holster, real kiss.
I also like Fobus;cheap and sturdy and light.
I have a Fobus paddle holster ( C-21)and a paddle (CU9G) for my cuffs and spare mag (made for a Glock mag but accepts my Browning HP mag perfectly).
http://www.front-line.co.il/ - product categories-kydex holsters-6th holster from the top.