Gosh lots of questions, I don't know the answers to all of them, but bumping you a bit. I'd start with a bit of googling and it might also be worth seeking some personalised advice from a financial advisor too as whether it's a good idea or not depends on a huge range of very personal financial circumstances including your age, marital status, any DC and their ages, employment/income etc etc. Money Advice Service and Money Saving Expert are my go-tos for 'beginners'. financial advice... this [[https://www.which.co.uk/money/mortgages-and-property/home-movers/stamp-duty/buy-to-let-stamp-duty-a26by5t8dmcb article) explains stamp duty.
One thing I'd say is that while it's not true that BTL is a totally unattractive investment, obviously the rental sector is still going fairly strong, all landlords have not suddenly sold off all their properties and caused a complete crash, but the combination of tax changes and also a slowing in growth or uncertainty in the housing market in some parts of the country means it's not the absolute money making machine it once was, and I think this has certainly affected the small scale, 1 or 2 property landlords who do BTL as a sideline to their main employment or income the most - if you are already a 40% taxpayer that's quite a chunk going to the taxman rather than your profit.
25% is normally the minimum deposit yes, although of course as with residential mortgages the lower the LTV rate the less interest you'll pay so the better the yield/ROI. Other costs aside from the deposit, mortgage, stamp duty if applicable and tax include: legal fees on purchase, agents fees (will vary depending on whether you want them to solely market the property or provide full management service), service charge and ground rent on the building (for a leasehold flat -watch out for swanky new build complexes with gyms, lifts, concierge etc, the service charge can be £££), maintenance of the property and any furnishings/white goods you provide including things like regular fire safety checks, boiler servicing etc, landlords insurance and a 'sink fund' to cover void periods in the rent and also if you get unlucky and get bad tenants that trash the place or stop paying rent and need to be evicted. All in all your costs can really add up and if you have a hefty 75% LTV mortgage you probably shouldn't expect to make much if any of a profit for the first few years (it may even run at a loss) so you would be relying on capital growth in the value of the property for a return on your investment esp if you then want to sell and liquidate your equity in the short/medium term (remember you'll also pay CGT if and when you come to sell the property).
Also, of course, BTL is an active investment meaning you have to actively manage your property (even if you purchase a full management service) in order to maintain your income, as opposed to a 'passive' investment like stocks and shares, pension fund, savings account where you just sit back and earn interest without doing anything. It can be quite a stressful thing particularly if you are the kind of person that gets emotionally attached to bricks and mortar - it's great if you have the time and skills to do some of the management work yourself as this can save a lot of money and increase the profit margin, but if you aren't local or don't have a lot of knowledge then you'll need to buy this in professionally which can really eat further into your profit esp if you get unlucky with your property or tenants...
I guess one thing I say quite frequently to people considering BTL is to think carefully about why you really want to do it - bear in mind that if you already own a residential property you already have a substantial amount of your capital invested into the property market - if your current flat is in London then doubly so as it would be the same market, so yes if London booms you earn double rewards but if it crashes you lose everything. If you have access to a 25% deposit on a London flat I am guessing we are talking a 6 figure capital sum - do you definitely want to put all of that additional capital into a market you already are highly dependent on, or do you want to diversify and spread your risk/reward out a bit? Other forms of investment such as making additional pension contributions, buying into a managed investment fund (which will diversify your assets for you, you can choose or more or less risky strategy depending on your own risk appetite), buying government bonds can be more tax efficient, certainly easier than BTL, may have less potential reward but equally are much less risky. Or if it is the housing market you specifically want to invest in, why London, is it because you forsee higher growth in the property market there than the rest of the country, have you identified a particular area you are confident will see huge growth, or because you or your DC may want to live there in future and so you want to have a property available - those are good reasons so fair enough if so (although you could consider other more passive ways of investing into the property market rather than directly purchasing bricks and mortar, e.g. buying shares in property investment companies or house building companies), but if your reason is that BTL is really the only investment you've ever thought about/heard of (no shame, so it is for many) you would probably want to do more research before jumping in?