So let's say I buy a flat for £80k. It brings me in £600 a month rent. Minus 55% for agent and tax. Minus 20% for other costs like surveys, fire certificates, insurance, etc. I now have £150 a month coming in for the £80k investment.
If you have a mortgage and interest to pay then that £150 wouldn't cover it. So no you don't make any money from the actual rent side of things. We'll you don't see a huge wad of cash in your bank account every month.
Your numbers look off, £600 per month is easily enough to turn a profit. One of the important mistakes you're making is applying tax to gross income rather than net profit.