I open the top open on mine if its in use and its mild during the the autumn winter and spring but in the summer I dont tend to need it. Not usually used as a raised bed? as it is where you put trays and or containers to germinate or for protection from the elements. So yes its a good idea to start off what you say you want to grow, but then they will become too big and need a larger place to continue to grow. Which can be a raised bed if you wish.

I agree with Drc a cold frame will not double as a raised bed. You grow things in pots or trays in a cold frame raising and lowering the lid to keep the temperature fairly even. We have several in which we overwinter bulbs and plants that don't mind the cold but do not like wet. Remember a cold frame is basically aluminium and glass.

A raised bed can be created in your garden by using bricks, boards or other material to surround an area of ground and filling with good soil/compost/composted bark. Usually to grow veggies.

We also have several raised beds using natural stone in, what we hope, look fairly natural shapes these have small alpines/bulbs planted in them so they are easier to see.

I have used cold frames in the spring/summer to grow Melons and Cucumber. If you dig a hole the same size as your cold frame, fill it with fresh horse manure, cover with polythene. You can raise many half hardy annuals from seed. Later put soil on the top for your Melons or Cucumbers. Don't let the roots grow through the polythene as your fruit will taste sour. In September you will have a good supply of compost for your veg. plot. The glass can be painted with cool glaze or you can have a mat on top to shade or keep frost out.

For salads in late winter, early spring, I think you would find that using fleece would be just as effective, and a lot cheaper.
For tomatoes and cucumbers, I'd use the largest polythene structure you can fit into your space. There are lots of 'mini-tunnels' on the market.
Apart from the warmth ripening the fruit earlier and for longer, above all tomatoes will be more protected from blight which is one of the real obstacles to growing outdoor tomatoes in so much of the UK.

Using cloches would help to warm up soil and get you off to an earlier start.