Heating things will be your biggest expense. Heating the house (assuming SO/Baby have their way) will be the biggest chunk of that. The old mantra of "insulation insulation insulation" is worth bearing in mind, but really should be replaced by "draft-proofing draft-proofing draft-proofing, then easy/cheap insulation insulation insulation".
If you have suspended wooden floors with an air gap below that is open to the outside, these are worth looking at. Lift the boards, insulated between the rafters, layer of boarding on top, relay floorboards. Insulation and draft-roofing in one go. Cheap to do, easy (ish) DIY over a weekend, and make a massive difference. Floor level has more of impact on comfort than higher up things, and will be in living areas that are heated more etc.
Also other drafts...get rid of them all! On a windy day, walk round with an incense stick, see where the smoke goes etc. Decorators calck, newspapar, old clothes, anything that plugs the gap.
Then for an electricity monitor, try your local library, mine lends them out just like books. Genius.
Going to stop rambling now, but also worth looking to see if you have a local "transitions" group. They will be a bunch of wannabe hippies, striving for an unachievable goal, but they will have a wealth of knowledge, skills and often share cake!