Look no further, here's the Byron DBY-22311 portable doorbell from Argos
It is easy to assemble and works perfectly. Just watch the video above and you'll get to see how good this portable doorbell is. It's totally wireless and no need to plug it into a socket.
Lots of melodies to choose from and not just your traditional doorbell sound.
Batteries are not included for the doorbell itself and it requires 3 AA batteries. But the remote or doorbell button got a battery, you just need to install it.
Last but not the least you can either use the double-sided adhesive that came with it or screw it on your wall or by the door. But what I did with mine was reinforced the double-sided adhesive to save me from drilling walls.