You have to appreciate you're asking to defy the laws of physics; can't be done. It doesn't matter how much time you have to waste
The current UK building regulations require walls to have a U value of 0.18. This means for every square meter of wall, and every degree Celsius difference between inner and outer, the wall loses 0.18 watts of heat. If the house is 10 degrees warmer than the world and has 100 square metres of wall, it will require a 180 watt heater on constantly to maintain the temperature
The best generally available (walk into
wickes and walk out with) insulation is polyurethane or polyisocyanurate board, more commonly referred to as kingspan or celotex. It had a lambda rating of 0.022 which means for every metre thickness of it, for every degree Celsius difference between one side and the other, it transmits 0.022 watts of heat
We don't use a metre thickness of it, but to save you the math, and assuming the wall were made of pure kingspan, you'd need 125 mm of it to meet UK regulations.
There are insulations available that are better, maybe even twice as good as kingspan - vacuum insulated panels and aerogels, not generally available and considerably more expensive.. but even if they're twice as good (transmit half the heat) you're still looking at 62mm of them to meet UK regs.
If you don't care about meeting regs of 0.18 then, using an estimate that you have 200mm of solid wall at a lambda of 1.75 and 5mm polystyrene at 0.033 adding the polystyrene will bring the wall from 3.5 to 2.3 watts per square metre per degree, or around 13 times worse than a new build house, down from 19.5 times worse than a new build
