Placement Year/Masters?

Hi there,
I'm a first-year student studying BA Hons architecture, originally from the outskirts of London but studying in a further away city in the UK. I have been trying to forward-think about my options. My brother will be finishing sixth form and heading off to uni in september 2026 which is when I graduate. I am aware that I will need to do a year in placement/get work experience in a firm for at least a year before I can continue to study for my Masters in Architecture. Ideally, I would like to do my placement in London and study my masters in a London University, and that is also most likely where my brother will be starting his studies, as we will be close to home. I thought that when it gets to that point I could start looking for an apartment in London where he could also live with me. Essentially what I am asking is, immediately once i have graduated, could i apply for my masters then defer my entry to the next year? are there any exceptions, or is that generally not allowed? Should I wait until my placement year is over before I apply for a masters? And how would my brother put down on his student finance application that he would be living with me? Do i still need to pay council tax etc? I'm just a bit confused about this situation, and would greatly appreciate some help. Kind Regards and thank you very much!