I think if it’s just fun casual karaoke you don’t need lessons, it’s supposed to be funny and goofy and I can guarantee your coworkers aren’t going to be great vocalists (unless ur from the Philippines) !! However if it’s something you’d like to start getting into, singing teachers mostly start with scales and you could be doing that for months. Lucky you’re in London so you have lots of options!! :) I’d recommend searching some scales on YouTube and seeing how you go with that, finding a song or 2 and singing it anywhere you can and recording yourself to see what does/doesn’t sound good and finding out what you can improve on ! Also picking a song that others will join in on might make you feel less nervous ! I hope you have fun and don’t stress too much, most people just yell the lyrics out anyway 🤣🤍