I used to live in the north (Bradford, white rose territory) I now live in Northern Ireland. I have built many a fish-house in the past each one superseeding the previous (learning by mistakes and adapting & incorporating new ideas) Pay particular attention to insulation the more the better. Incorporate more in the roof and build a sub floor and place some insulation under it as well. Once you've that sorted out you can then go on to address the heating aspect of the whole thing. If you have insulated well then you can space heat relatively cheaply using fan heaters coupled to thermostats. Even with this method your top teir tanks will be a few degrees warmer than the ones nearer to the floor, so you will have to choose your species to suit. For sucess in keeping and breeding killifish, water conditions are paramount and I would strongly suggest you buy some corrugated perspex sheeting and attach this to your fish-house roof. This will allow you to collect rainwater via guttering and into a water butt. This water can be mixed with tap water and tested to gain the right conditions for a particular species. I bred many a species using this method including some of the more demanding killis like Rivulus Xiphidius.
With that size of fish-house why not incorporate a pond, make a box from 2x2 and plywood and insulate this with polystyrene then add a pond liner (light blue allows you to see the fish rather than the normal black) I had these in my fish house, the ponds are excellent for growing fish on. Panel around the sides and top edges to make it look smart.
Another good idea is and air pump linked into a ring main. buy some 1" plastic overflow pipe and four plastic elbows. to fit inside your structure at roof level. place you pump a one end of your fish-house and drill and tap into the pipe using airline connectors and silicone. then you can bleed out anywhere in the main into your tanks. (Ideal when using sponge filters) Provision will also be needed for you foodstuffs ie: hatching brineshrimp. microworm,grindalworm etc,etc,etc. I have included just a few tips here. if you need any help on specific areas of Killi keeping please let me know.