Modifying some household electrical circuits is still acceptable and within the scope of works that doesn't have to be declared through a Building Control application. The scope of what you propose to do is acceptable.
I am sure you are talking about a small computer router, powered off a double-insulated AC/DC converter of around 10W output? (and not a power tool router - just checking). If so the lighting circuits (normally protected by a 5A fuse or a 6A MCB) are more than adequate to cope with this. The main difficulty you have is that the AC/DC converter is designed to plug straight into a 13A socket. So whilst you could mount a 13A socket up there as a spur off a lighting rose, it must be protected by a fuse of 2A. And you must take steps to ensure that no-one else ever takes the AC/DC converter out and puts a heavy electrical load into the 13A socket.
You could do this by purchasing a switched spur plate like this.
http://www.tlc-direct.co.uk/Products/MKK1060.h tml fitted with a 2A fuse, and running the feed from the ceiling rose through this, then onto the 13A socket. For extra clarification, put a label on the socket warning that it is for maximum loads of 2A.
In my opinion this is safe, but someone will probably tell me it isn't good practice to do and not within the guidelines of BS7671.