A lot of plants can be sown outside-most hardy annuals where you want them to flower this year-you can raise a lot of hardy perennials from seed they do not need heat-but will probably take 12/18 months to flower
The biennials -like wallflower sweet williams etc can be sown outside in April-will flower in 12 months
It is the half /hardies that are trickier-they will not tolerate frosts but these can often be started indoors just on windowsill- but would not start those of for at least a month or so
You are not so limited as you may think-these are just some of the options