If you're getting married in the UK, rain is a likely part of the proceedings. Make sure you pick a venue that has a beautiful indoor alternative if you need it. Most venues in the UK plan for exactly this — if you're opting for an outdoor space, this often includes a conservatory or adjoining room where your guests can keep warm and dry if the weather turns. That space should be as beautiful as your original location, but double check before signing on the dotted line that you're not paying for bland and ugly conference room for your plan B.
If you have your heart set on a wedding in the woods, you're going to want a barn, or at least some canopies in the trees, where your guests can shelter. If you're determined to have your ceremony outside no matter what the weather, then it's down to you to keep your guests dry and warm. They'll need umbrellas, waterproofs, and stiletto protectors to stop high heels from sinking into the mud.