Can I add an outlet for electronics to a lighting circuit?
No.

There are a few issues here. While what you're doing could in theory safe, the rules are designed so that "what if"s are taken care of. ie. you're installing a socket which could be used for anything, so you have to allow for that.


Sockets now require RCD protection, which you won't have on your lighting circuit.
The rating for the lighting circuit will be lower than a socket circuit. 
You're required to terminate a lighting circuit in a suitable fixture. The rules allow for "a suitable power socket", but the implication here is that you'll be using a type D socket, as is typically used for things like table-lamps/uplighters on lighting circuits. Adding a type D plug to your appliance would probably invalidate it's certification.
AFAIK you'll be required to submit a "Minor Electrical Installation Works Certificate" to local building control. While this can be done by a homeowner, it requires you to be competent, comply completely with wiring regulations and have tested the works properly.
Your insurance company might require that works be carried out by a qualified and suitably registered electrician.


But! But!
Ok, so for point #1 you could add an RCD socket. 
For point #2 you could in theory add a FCU (fused connection unit) with a 3A fuse - but here you run the risk of someone coming along in the future and changing the fuse for something higher. This in effect rules out this potion
Points #3-5 I don't see a way around.

All-in-all - this has bad idea written all over it.