Wooden cold frames come in two main designs; glazed to the ground and those with solid wooden sides.
Providing your budget will stretch to it, a wooden cold frame with solid sides is better suited to the British climate. The wooden sides absorb heat during the day and then radiate it at night, helping to keep nighttime the temperatures up in both the early and late growing season. They tend to be so effective that with a bit of carefully planning you could eat home grown lettuce most of the year round
If you don’t intend to start your growing season quite so early or extend it quite so late, then the cheaper fully glazed cold frames are an excellent buy, and they look pretty good too.
Hawthorns make a great range of cold frames; they are great looking, robust and highly functional. Being made from solid pressure treated wood they will last without the need to repeatedly apply preservatives. The solid wooden frame also acts like a radiator, storing heat during the day and releasing it slowly at night, keeping your precious seedlings warm.