Do you have a contained cabinet or room within your loft or is it the entire loft? What I did with my grow room was to install a small extractor fan (for a bathroom)... only I flipped it 180° so that it pulled cool fresh air in from outside (instead of extracting air)... using this kind of fan allowed me to add an exhaust to it and feed the exhaust out a window... you could do something similar in your loft but feed the exhaust to the edge of the loft where you should see venting from the roof around the outside. That way you'll be able to pull in fresh air from outside. Alternatively, just feed the exhaust through your breather tile if you can. Additionally, if you then have a separate vent in the grow room, having this fan installed should naturally push the stale air out of the room. This will also help any new plants gain thicker stems as there will be a constant air flow over the stems.