That knocks out most of the obvious options. Probably the largest factor after flying and driving is how many people occupy a given building. However, as you're renting, you're probably excluded from taking a sub-tenant even if you have a spare room. Might be worth asking the landlord if you can add extra insulation (assuming that you have access to the roof space and there's less than the recommended 8 inches). Many landlords will be okay with changes like that as long as you ask permission first. AS long as you don't want floorboards in the loft, there's grants that pay most of the cost of having loose fill added to increase the thickness of loft insulation.

I'll assume that your hot-water tank is well lagged as that isn't a structural change.

Shopping can make some differences. Always be aware of food miles. Buy stuff produced in the UK whenever possible and try and be aware of stuff that has been freighted by air. The more local the better is the rule when it comes to food. Farmers markets are good if there's one you can get to.