How do I bathroom / kitchen / extension? etc.

We're moving at a snail's pace as we only spend the occasional weekend on site and we're trying to source materials on-the-cheap.

Envelope upgrades list
Loft: 400mm loft insulation 0.1 W/(m²K)
Sloping roof: 30mm PIR between rafters, 150mm below rafters 0.15 W/(m²K)
Eaves: 400mm loft insulation 0.1 W/(m²K)
External walls: 180mm PIR internally gapped away from solid double bricks 0.15 W/(m²K)
Floors: 400mm loft insulation 0.1 W/(m²K)
Double glazed low-e argon sash windows 1.2 W/(m²K)

Ventilation list
Airflow front-to-back via cleared under ground floor (10 bricks deep)
Add more air bricks
Airflow vertically into newly gapped external walls
50mm above sloping PIR
Cold/sealed eaves/loft
Block chimneys
Air tight tape everywhere
85% bathroom/kitchen heat recovery (4x single room units)

Imagined heat loss reduction (assuming 150sqm envelope, 0°C outside, 20°C inside):
Losses before: Avg. U-value of 2 W/(m²K) = 2 x 20 x 150 = 6kW
Losses after: Avg. U-value of 0.5 W/(m²K) = 0.5 x 20 x 150 = 1.5kW

Ventilation losses are more difficult to guesstimate, but we'll aim for a 85% improvement everywhere.

Switching to PV (planning willing) or an overnight electric tarrif for a Sunamp would reduce the cost of our hot water by at least two thirds (nice-to-have).

The budget for renovations, plumbing and 3 new bathrooms is non-existent (£25k), so we're trying to secure the cheapest price per square meter for insulation and are doing the work ourselves. An AS heat pump is another nice-to-have.

Once the renovation work is done, we'll improve the kitchen. Currently there's a dodgy rear extension which we'll remove and then re-build including a side-return (planning willing).

The last two weekends we broke the back of the loft rooms (attached). We've been very lucky with the condition of existing materials so far.