Our house is cold in the winter, but doesn't have cavity walls, so we can't insulate it that way. Jenni has found this stuff called "insulating paint additive", which apparently contains "NASA developed technology".

Does anyone have any experience with it? Does it work? Is it snake oil?

Absolute snake oil - at least for your application.

Look at the heat transfer mechanisms in play - conduction and air transfer are the major factors in a house. Radiative is minimal.

They *do* insulate well against radiative - so tests can show a reasonable performance cf rockwool etc. But in situ it's just not relevant. There are similar 'foil' linings for roofs - the consensus (including self-builders who've used them) is that they don't work. If this stuff did work then it would be used in applications like fridges + freezers. They still primarily use 'foam'.

I've been looking at this for *years* wrt building my own house. I've found that to make your house warmer :
* Identify draughts and block them - doors, windows, gaps in the floor (depending on construction). : this is the single best value action. A few pennies on draught exclusion can save a fortune. Of course it may already have been done.
* Loft/attic insulation : next best value job. Get a pro to do it - in the UK there are grants. http://www.energysavingtrust.org.uk/proxy/view/full/2019/grantsandofferssearch
* Cavity wall fill : not applicable but the next best
* If you are lucky and have good access under the floor you can fit insulation under there. Obviously not for concrete

After that the payback isn't worth it

Occasionally you get an opportunity to renovate and you may want to lose 4" off an external wall by using plasterboard over battens with well fitted insulation.

I don't know what kind of insulation is commonly used (or even available) in the UK, but if you can get it, polyisocyanurate boards would be more space-efficient than your (I assume) fiberglass suggestion.

I would add: make sure that your system has a mechanism (system humidifier, say) that will maintain a reasonably high indoor humidity -- increase heat-carrying capacity of interior air, improve heat distribution through your spaces and reduce heat loss.

I don't have any idea of what optimum levels are, but I just fire up a full tea kettle when I want to warm things up a bit.

As a past and possibly future resident of a boat, I dealt with the reverse problem -- forcibly venting interior air to the outside to take humidity with it.