I don't know if there are any legal restrictions in your area. The main practical problem with large tree near a house is with the roots, which can damage foundations, and enter drains. The general rule of thumb is to plant the tree at least one and a half times as far from the house as the normal mature radius of the tree. I.e., if the tree would normally grow to 13 meters across, then plant it at least 10 meters away from the house.

There is apparently something called 'right-to-light' and may be worth looking into. Have a look at the following, there is a pdf file. Have a word with your neighbour if possible or your Citizens Adive (if you still have one in these days of cuts).
http://www.rics.org/en-EUR/knowledge/consumer-guides/rights-to-light/

The main danger from willows is the roots going into the drains. They are experts at tracking down the nearest water supply!

You can only claim right to light if you have had uninterrupted light for 20 years before the obstruction appeared, and there are precise rules about how much light is lost before you can claim.. It is more usually claimed with regard to extensions.

Call your local Council and ask. Only they will have the answer, as rules and regs vary depending on location.