Quote Originally Posted by cannabis-seeds
People in the UK commonly either go passive or use fans on the intake & exhaust to the loft/attic - they seem to want as much fresh air as possible.
+1 on venting the grow room to the attic - it should mask the heat signature - even attics in the UK are warmer than the lower levels of the house. This would also shunt any smells up into the wind.

Using florescent lighting will keep the heat down enough that any FLIR equipment would not notice any anomalies. Yield may come up a bit short of what HID lights will give you but will also give off much less heat. I have a pair of 2' 4 bulb florescent light fixtures that I use during the summers here that hardly give off any heat as compared to a 400w HPS fixture. The two florescent fixtures use 200w but give off much less than the single 400w HPS and ballast. The HPS also more easily burns leaves and bud. With the florescent lights they have to be touching the light bulb to burn the leaves.

Large grow operations commonly use 2000-4000w and more lighting which may cause a heat signature on the outside walls of a structure not well insulated. A small grow operation using no more than 400w of florescent lighting might not require more than a small fan in a window moving air across the room and expelled via another window. Heat will be minimal. Make sure any florescent lighting you buy is high output. Use 6500k for vegging and 2700-3000k for flowering.