I think Andre’s covered this 
In case you’re interested though, there is a new service called Curl that’s due to launch it’s beta pretty soon which lets you pick any username that you want. Whether it can gain traction & reach enough scale to be useful is the big question of course. But in case it does..you can reserve your username here 
that’s not a referral link btw 
Also, RichardR’s shared some details on how much you can customise your Monzo.me username here.