Hi Eloise,
For your grass I would definitely go with a kentucky bluegrass/fine fescue blend. If you can get it on the ground soon then you can catch the cool spring window and the rainfall do most of the work.
I wouldn't do wildflowers unless you want a really wild/taller look. Maybe do them in a an area where you wont mind that so much.
Sweet woodruff is a ground cover that loves moist loam, and some people eat it I think lol. You could mix in other ground covers in sunny spots, like brass buttons.