I sowed some Calendulas now as an experiment- I have loads- taken off plants last year.They were in the cold frame but have now brought them in to my kitchen windowsill, a couple have popped up. Will sow some more later this year as well. In my experience here they are pretty tough so once up they will be alright
I would wait a while till late Spring/early Summer to sow Larkspurs outdoors,just based on my patchy experience of them.
I've been growing from seed for only 2 years and relatively speaking the last 2 Winters have been fairly mild here. I start mine on off in a unheated (Wilkos)propagator on my kitchen windowsill with the radiator on underneath to give them a bit of gentle bottom heat. I have 2 coldframes/ sturdy mini greenhouses in a very sheltered spot which I then grow them on in.
So far I've had really good results sowing in Jan- my garden full of snap dragons which are still alive for one. I even managed to grow Lobelia from seed started off indoors Feb/March and then in my coldframes.
I feel the cold so have my heating on all the time in Winter but just on a thermostat, works out cheaper that way rather than having to heat the house from cold all the time. For me I don't think it makes any difference how many rads are on, in fact the system I have here, it might work out better as the whole house is warm.