We own four btl properties, plus our own house. Two of the btl properties are flats, but all four have mortgages on them. None are in London, but are spread over East Anglia.
I really don't think you have factored in all of the costs which can beset you with this plan.
You will have mortgages, as you know. However, flats are very often owned on the leasehold system in the UK, and London is no exception there. You could be paying ground rent, and you will be liable to pay service charges to the freeholder or their managing agent. There can also be sudden and unforeseen ad hoc expenses for things like external repairs, repairs to communal areas, roofing etc. These can be significant expenses and would very likely knock your budget completely sideways.
Also, for the life of me I cannot see how you will squeeze two flats out of that tiny deposit (by London standards) unless you take out huge mortgages.
Interest rates have been at rock bottom for so long now that I think many of us have got very used to that. I remember when they were controlled by politicians rather than the Bank of England though, and were much more volatile (15% at one point under Thatcher). They are more stable now, but will begin to rise again at some point, possibly in the next couple of years. We are aiming to be virtually rid of mortgages by the time that really becomes an issue, but you will have taken on some new ones.
I'm not saying don't do it, but rethink the jacking in work at 40 lark. You could regret it.
Perhaps one alternative possibility would be to sell your house to release all of that equity and invest the profit in some proper pensions and ISAs, both of which can give you some income in the future whilst maintaining much of their capital if wisely invested.