Why is my house cold?

How fast the termperature rises in a given room depends on a lot of factors. Is the radiator the right size for the room, for example? If it's too small, then the room will take a long time to get hot. Is the boiler itself set to heat the water in the CH circuit as hot as it can be? Mine, which is a Vaillant has dial on the front to set the temperature of he water in the CH loop, turning that up from 60C to 70C makes a big difference. You have said there is insulation - 300mm in the loft is a good depth for the fibre roll stuff, do you have that much? My home is a bungalow so this is quite a big deal for me in keeping it warm, there is a lot of ceiling area.

I think at last you are getting down to the cause!!

As already posted your rads should be hot in ten to fifteen minutes after switch on. Without that there is no significant heat being output to your rooms.

So you now need to find the cause. Not unlimited to the following but a few simple steps of diagnosis may help pinpoint:

Before starting check that you have a small header tank and that it has water in it and that the ball float valve is working (depress slightly and water should enter)....set the programmer to on, temperature control on boiler to high and turn up any thermostat (s). If separate controls select heat only and not hot water for now.

1 boiler giving sufficient output. Seems ok by design (earlier post) so check the temperature of the output pipe. Should be too hot to touch so if you do not have a temperature meter feel how soon it rises to a too hot to touch temperature. That will be about 50 -60 degrees Celsius and should be there within a few minutes.
2 circulation. Can you hear the pump running? (Assuming is pumped?). If you can get access to pipework further round the system the temperature should rise there too, a bit slower and maybe not quite as high (depending upon if heat is being dissipated but as you are not getting much heat output it should rise quite quickly and nearly as hot as the output from the boiler). That should confirm at least some circulation. If the pump feeds a hot water cylinder too and the water in it is cold then that will take some heat and slow things down. For house heating try to run it without hot water heating at the same time -note initial comment on control settings).
3 if all that is working but a bit slow (say over five minutes take note of a previous post and if possible you can turn up the pump speed, for now.
4 Do any rads get hot? If non it could be the heating valve that is a problem(not opening) or the electrical control to it. Presumption, you have one fitted!
If previous steps are positive then it suggests that the rest of the system (boiler, circulation, water level) is fine. There is often a bypass route for the circulated hot water. The latter might be through a single radiator.
5 for each radiator, check that the valve is open and that the flow control (often a screwdriver adjustable valve or possibly a settable temperature valve) is also fully open.
6 if rad still cold entirely or in part try bleeding one to let out gasses that should be replaced by hot water. Use a cloth to catch drips and partially open the bleed valve with a bleeding key sufficient to let the 'air' out but do not fully unscrew! Close as soon as water appears. If air comes out then that will be replaced by hot water and the rad should heat. Repeat for all rads.

Rads cold or cooler at the top indicate a gas build up, cooler at the bottom indicate a build up of gunge!

If all goes well you ought to have heat. If not then further investigation will be necessary.