I just start them on a south-facing window sill. I've never used any kind of heating, even for the seeds that the packet says do best with heat. Just stick too many in a 4 inch pot with seed compost, cover with cling film and leave in a tray with water in it - never water from above, mostly because it would make a mess of my window sills
I think the only seeds that have ever given me problems were meconopsis and rosemary. I persevered with the first, and visited a garden centre for the second.
I gardened in Finland for years and the plants weren't supposed to go out until the first week of June - mid-May in a cool greenhouse - though I generally got impatient: such a short growing season. I have so enjoyed putting them into a friend's greenhouse in March here in Suffolk, and now my own polytunnel!