Were I to enter BTL it would need to be:
1. An outright cash purchase.
2. In a desirable area with a sufficiently affluent demographic.
3. Preferably not a flat.
Someone I work with has several BTLs (of varying type / standard) acquired over the last couple of decades. Now she's over the 'equity hump' that will be her retirement sorted which is great - but along the way, I would have lost the will to live. It's a dreary and daily grind; an ongoing minutiae of tedium.
I think the best advice (above) is: be realistic: unless your plan is guaranteed to get you to the place you want (in London), look for somewhere else you can afford, buy, live in - and be happy.