UK houses are some of the most poorly insulated houses in the world. Good insulation is the key to a low energy house. Leave no exterior facing surface uninsulated :- top, bottom, walls, doors & windows.
​Your main goal should be to  -
NOT TO CONSUME ENERGY IN THE FIRST PLACE.

Shown above before and after external insulation.
​Start with insulating, you be amazed how much warmer it will feel, saving on the heating bills.
The good news is there are government grants available

Thermal Mass

The amount of stuff INSIDE the insulated area.
An important concept to understand when insulating and the reason I like EXTERNAL insulation and not INTERNAL insulation.

If you had 10kW of sun power going into Zero thermal mass you would have box that's get hot very quickly and cold as soon as the heat source has gone. An uncooled solar thermal panel can reach 200c !

External wall insulation is best as the bricks, stones etc absorb the heat during the day and release it over night making the temperature much more stable. I know insulating over a lovely stone cottage is going to be unpopular but we all have to change to reach net zero.

External vs Internal Wall Insulation

External Insulation

Pros

• Much greater thermal mass
• Easier installation
  
• ​Installation is on the outside so does not affect the inside of the house
• Can improve the look of the house
• Grants available

Cons

• Can be a bit clunky looking on the outside detail

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Internal Insulation

Pros

• Does not alter the external looks

Cons

• Loss of room space
• Thermal bridging
• Low thermal mass
• Difficult to install
• Building work in your house

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Loft Insulation

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Roof  Insulation

Bubble wrap type insulation is not the best probably in terms of thermal efficiency, but it still managed to raise the internal temperature 10c !
If doing again Id go for the multi layered type like ThermaQuilt

I Installed this during a cold snap (-15c outside) because my pipes had frozen.

Temperature in the loft went from -5c to +5c

Insulating your loft

Cost £300
Can be done in a day by a competent D.I.Yer

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Under floor insulation.

   Insulating under a suspended wooden floor is tricky if there is no space to work. It involves removing the floor boards and skirting boards. Insulating from underneath is much easier as you do not have to disturb the floor - Skirting - Decoration.

   The main aim is to stop air flow from the cold under side to the top so make sure you tape all the joints and trap the edges with the skirting board (or battens if insulating from below).

It can be either stapled to the sides of the joists or clips shown.