Hi awilliams,
To keep it brief, here are some areas I would check:
Drafts
Rather than insulation, drafts are the main cause of a house feeling cold. Mitigate the drafts and your house will feel a lot more comfortable. I speak from experience as I live in a detached bungalow with no cavity wall insulation. I've took plenty of measures to rescue drafts and our house in now a lot more comfortable.
I can see from your pictures that the area around the front door and the intersection between skirting and flooring the temperature is a lot colder. This is probably due to drafts. Check the seals around the front door and replace them if they're worn or broken.
It's also worth checking all your windows as well.
Have you got a suspended timber floor? If so, it's probably wise to check all the gaps around the perimeter of the floorboards. Normally there's a 10mm gap for expansion. You could use a flexible expanding foam to fill the gaps if there are any.
Also worth checking any gaps in the skirting boards. If you find any, use caulk or filler to seal them.
You can go a step further and make sure all downlights, ceiling roses and extractor fans are sealed into the loft cavity. I used down light covers in our house.
Radiators
Again, speaking from experience, its worth checking your radiators are the proper specification for the rooms. There are a number of online calculators you can use. Essentially you have to take into account the cubic meterage of the rooms, how many windows, how many external walls and what's above the room (loft or another room). Then you can calculate the required BTU (British Thermal Units) the room is going to require to heat and match a radiator accordingly. We found that most of the radiators in our house were under spec and have since been replacing them one by one. The time it takes to warm the rooms, and the amount of time the heating is on has been greatly reduced.
Insulation
Short of having your cavity walls insulated, it's worth checking your loft insulation. I believe current regulations stipulate a thickness of 270mm in rock wool or fibreglass. Check to make sure you have got full and even coverage.
If you have got suspended timber floors, it might be worth considering floor insulation. One method is to hang wool insulation between the joists. I did this in my house and it's certainly made a difference to comfort levels, especially with a vinyl flooring.
If you've got any further questions, feel free to ask.