Hi Jane,
I think your covered raised beds will work exactly like a greenhouse - a greenhouse, after all, is just a layer of glass or plastic between the plants and the elements!
You could always get a couple of thermometers and measure the respective temperatures of your beds and the outside world - you could post the results on here over a few weeks period, it would be interesting to see how much difference there is.
So, bung your seeds in and see what happens.