We let a property out (because we intend to move back in eventually). It is NOT a good financial investment at present (if we weren’t planning to move back we would put it on the market tomorrow):
London flat property prices are actively falling - in the past landlords made money because the value of the property went up over time. That isn’t true any more. The value of our flat peaked in about 2018, and has fallen by about 5-10% since then. My brother’s flat is on the market, and is worth less now than he paid for it in 2014 despite being fully renovated since then.
As you mention, there is stamp duty, income tax on your rental profits, CGT when you sell.
Most councils now have, or are in the process of introducing, a register for landlords - which costs about £800-1000 to join, and mandates various safety and energy efficiency standards. So there will be a cost getting it up to standard (normal residential properties don’t need any of this).
There are the annual gas safety certs, EICR every 5 years, etc etc. Property management fees unless you are going to manage it yourself (if your lifestyle won’t allow you to respond to a midnight phone call about a broken boiler, don’t manage it yourself).
And then there is wear and tear. Tenants will not look after things like you would. Appliances will break about twice as fast as the ones in your house do.
Expect to replace the bathroom and kitchen every 5-10 years (£30k for both), the floors every 5 years (£5k), repaint every 3-5 years (£3k), assuming you want half decent professional tenants. And obviously any major works on the outside of the building - as joint freeholders, we have just been stung for £3500 for the downstairs neighbour’s basement flat damp rectification. We know we need to do work to the roof/gutters/soffits next summer, which means putting scaffolding up, which means similar costs. The boiler is 15 years old. The list goes on.
Our flat is a lovely two bedroom period maisonette in Brixton, in a very good state of repair, worth about £500k. Rents for £2k pcm. After maintenance costs, tax etc we are actually receiving about £5k per year net profit (we are additional rate taxpayers but if you are buying a BTL I suspect you are too). I’d make more than £5k leaving £500k in a current account, let alone actually investing it. As I said, we aren’t trying to make a profit, but if you are, a BTL is not the way to do it these days.