There are so many uses for birch. The leaves crushed in your hands with a little water create a natural soap which is a mild antiseptic. It makes excellent firewood and is terrific for carving.
It is possible to extract the sap, which is wonderful to drink and can be used to make a wine, by drilling into the tree and entering a snug fitting tube above a receptacle which can collect up to a litre of sap over night. Only do this with a large mature tree and do not do it unless you have the landowners permission first. If you do not fill the hole afterwards the tree will bleed to death so it is important to fill the drill hole with a debarked piece of living birch that fits snugly into the hole. This will stop the tree dying. Ray Mears does a very good demonstration of this or there are plenty of others examples on YouTube. Hugh F-W has a good recipe for the birch sap wine on his website http://www.rivercottage.net/recipes/ben-laws-birch-sap-wine
Enjoy these trees all over London's parks and streets. Pictures above taken today in Brockwell Park.