The issue is you need to design for 1-2 weeks of little to no wind and very little solar in winter, when demand is it its greatest. This is what is so tricky, unlike more southern climates where you have pretty reliable solar year round (even if its reduced, its not close to 0 which is the case in the UK). Instead of storing a day or so, you need to store weeks of energy.

Also you're forgetting residential electricity is maybe 1/3rd of total demand.