Heat and leaf questions

Haya. Couple of questions I hope you guys can help with.
First time I'm gettign a decent indoor growth since cross-over! :D Outdoor grower been forced indoors!
Got a 500w 3 buding plants (about 3 weeks to go) and just put in two smallers, cuttings grown under flos. Space about 2meter wide by 1 long, pots about 10ltr. Everything is inthe attic and the temperature is dropping!!!
Good floor and ceiling insulation but the temp is 13C and gonna drop, UK. Was told to have a small desk fan on constant for air flow, but with temp drop, lights at night, worried about the temp drop? Ideas???? The total area to work with isn't much and worried about leaving a heat source up there...

One of the plants has suddenly developed lime green leaves all over. Shes not taking her water, usual 2litre every 4 days, and has seemed to slow down. Shes on the same time, about 3 weeks to go. Think its a Thai(where canI go to compare leaf variety?
Had a white fly infestation for a while, but all gone, too cold I think!!
Cheers. :o

im in uk and have to suffer the winter aswell, i had a crop last year that was well cold during the night, colder than 13 deg. and they worked out well, you probably wont get the optimum amount of buds but i dont think it will kill it.