lowering the height of a large silverbirch

Removing the top from a birch will result in an ugly tree. There will inevitably be a lot of haphazard regrowth around the top.

A much better way to let light into the base of birches is to remove lower horizontal branches.

I have friends who have their birch trees topped and thinned regularly to reduce wind resistance as they were planted too close to the house and are a potential danger in strong winds.   A good tree surgeon will do it without making the tree look ugly.

You should be able to ask for photos of trees he's done or else a list of his clients whom you can call.    Don't leave it too long as the sap will be rising soon, if ithasn't started already, and your tree could bleed to death.     

But birch sap wine would be an option. 

There is a certificate of competence a good tree surgeon should advertise as having. Can't remember what it is but if you look at yellow page adverts you should see it.

A lot of people advertising as tree surgeons  or tree care specialists have H&S and chain saw certificates - this does not mean that they have the expertise necessary to shape a tree - you may find some help here http://arbtalk.co.uk/forum/homeowners-tree-advice-forum/