Improving loft insulation is generally the easiest and least expensive thing to do, so it might be worth doing that even if it doesn't make that much difference.

Your windows appear to be the biggest heat loss, although replacing with improved ones may not be cost effective. Perhaps consider secondary glazing inside, or thermal curtains, blinds or shutters.

Your doors seem to have the worst performance of any component, so looking at improving those might be worthwhile. When I built I considered using a conventional outside door for weather resistance and then making a separate insulated door inside it. But in the end I just went with 'normal' PH doors (too lazy).

Do your heat loss calcs match your bills (or electricity logging)?

I would top up loft insulation to 500mm , at least to 400mm

Draughtproofing on the doors should be cost effective too.

A few thoughts, having skimmed the above:

- losses thru the floors aren't shown
- looks like you've assumed -15oC externally for the calcs condition, giving a heat power input of around 3.5kW, let's say 4kW when floor slab added in (just very round numbers, as it's all assumptions anyway)
- if you improved the loft insulation to say 0.1W/m2K, with say another 200mm of insulation, you'd reduce the heat loss (power) by say 150W, so roughly 4% of your heating bill, say £20/year, even £40/year when your new elect tarrif comes in to play. Is that worth doing, for a £40/year saving? I'll let you determine that. Personally I probably would, but that's what I do for a living.
- concrete walls are an obvious one to tackle, based on the Uvalues, but you're unlikey to trim even 50W power losses, so payback would be ridiculous, for stripping walls and over-insulating
- if you particularly want a porch, then build it, as it can be a usefull addition, somewhere to stamp off snow, stop the inrush of cold air, but financial viability of several £k's versus minimal savings?

I would hire a thermal imaging camera, and look for draughts, thermal bridging, weak spots, as these "could" be simple fixes, with actually quite a good improvement proportional to the effort, and can be sources of condensation generation, and pretty much ignore the large main elements listed above.

Shutters/thermal curtains for the glazed elements (per DJH) are an easy win (ie. not disruptive) but the costs of these can be huge. Consider the carbon footprint of additional materials and human effort/transport, versus the almost zero carbon of your elect supply.