Sorry but I don't think you can compare the return on owning the property long-term to investing £200k in a SIPP.
Er, that's kind of the point of the thread. I'm happy to make the argument.
London house prices have gone up 72% in a decade! If you sell and put the money in a SIPP, you'll miss out completely on any capital gain.
How on earth did you reach this conclusion? Do you understand SIPPs? If OP puts the money in a SIPP she/he can choose the underlying investment (the 'self invested' bit of 'Self Invested Personal Pension'.) Some investment choices will be focused on capital growth, others on income, others will be a balance. I happened to pick F&C Investment Trust because it's mostly about growth with a small income, which mirrors what OP is currently enjoying on the property. Other investment choices are available. FCIT has risen by 63% in a decade, which in a SIPP or ISA would be free of CGT, while the income would be tax free too. London house prices may well have gone up by 73% on average, but by definition half of properties will have risen by less, that gain will be taxable for OP, and one property is considerably less liquid than an £11.30 asset traded on the London Stock Exchange.
I've owned properties on interest only mortgages - as long as you're covering the mortgage and making a bit of a cushion for repairs, that's fine IMO.
If OP wants to leverage her/his investment then there are plenty of ways of doing that. The risk is that it amplifies losses as well as gains. I'm old enough to remember negative equity from 30-35 years ago, which you seem to discount on account of your Panglossian optimism. I do not.
You won't make much more in an ISA and you could make a LOT less if property prices stay high.
Oh dear....
And let's face it, the UK is not building enough property, the population is increasing, everything points to property prices continuing to rise.
Apart from affordability, government policy, bank credit availability... Seriously, while property prices may hold in absolute terms anyone who thinks the property price inflation of the last twenty years will continue in a straight line is utterly deluded. Even rank amateur BTL landlords are having second thoughts.