Can you help? I am looking for a light to fit my sub-compact Glock 27.
Absolutely! Your Glock 27 will need to have the Streamlight TLR-3, part number STL-69220.
Can the streamlight TLR-1 mounted light fit onto a H&K USP 45 cal without any adapter?
No; the H&K USP requires a rail adapter in order to mount the Streamlight TLR-1 Weapons Light.