I wouldnt worry about that in the UK, our police are far too bizy doing speeders and filling in paperwork to bust small time grows, a decent grow might interest them tho.
My car got nicked the day after my last harvest, police invited themselvs in at 4am to take my statement, they noticed the fresh cut smell and gave me a grilling, it was obvious what i had been doing but they never took it any further, it was on a plate for them. Needless to say i shat myself.
On the subject of heat and flir, if you grow in an attic (like i did) its gona be easy to see, snow & ice melts away so quickly above the heat source, it would show up on flir but in an insulated house with heating etc a 400-600w bulb will only moderatly increase the room temp outside the grow area.