I had a shared ownership property in the past.
Things to reflect on. We got out when the rent and mortgage ended up being more than a full mortgage was. Initially both combined has been really reasonable and we couldn't afford anything else. We staircased out. This in itself was a bit of a nightmare with various issues with legal bits and pieces and mortgage companies. If we'd left it longer the increasing rent issue would have made it impossible for us to save to buy it out and also unsaleable to others as the numbers would have been dreadful. We'd have been trapped.
The rent is your issue and onward sale is your problem.
We found ourselves at the mercy of your rent always going up and having no control. But I caveat this too. If you are only ever going to be able to rent then it's not a bad idea for a very secure rented long term property because you are going to be stuffed with rent increases anyway but you aren't at the mercy of landlords being dicks. But you are responsible for upkeep of the property yourself.
This is great - until you have some kind of change in life circumstances and you find the property too small or you want to move to another area or you want to just sell for whatever reason.
Then you can find yourself really trapped. You can't negotiate with a buyer on sale price - the sale price is fixed and is set by the organisation who own the other part of the property not you. If it's priced right that's fine. If it's over priced you are screwed and it's very difficult to get them to agree a reduction in sale price. The worst case I've heard of is a property on sale for 4 years because the housing association would not recognise it was over priced. Remembering of course it's already got a smaller potential pool of buyers cos it's shared ownership. Be VERY VERY aware of this in a housing market where the market value of your property is likely to drop. That would be a concern for me with London too.
And of course you can't let it out because it's subletting - which you don't have a problem with full ownership. So it can be restrictive for alternative options if you need to move quickly.
I'd also say the risk is even higher with flats because you aren't just stuck with rent increases, you also have service charge issues and potential issues with leasehold issues on the building where the freeholder can make you pay.
Shared ownership also seems to work for people who are willing to use it as a quick stepping stone - there for 3-5 years and then move onto something full ownership. Again it's the security of owning rather than renting that's the good thing. This means you aren't stung by the rent getting out of hand in increases.
So my point is WHY are you considering buying shared ownership and what's your long term plan? How likely is it you will get married/have kids or otherwise need to move?
If this is your forever home or if it's your right now but out a few years then yes it might be ok and worth considering.
But my crucial point is if you don't understand the principle of compound interest increasing your rent don't even look at it. This is absolutely essential to understanding what you are actually buying.
Overall I would say shared ownership worked for us. However I fear we are the exception that proves the rule of don't touch shared ownership. Our combined income had more than doubled by the time we got out, we made zero equity on the property - we bought in 2007 sold at what we bought at after well over a decade - and we still struggled to get out. I loved our house for many reasons. The area was great. But... Yeah... I'm not convinced I'd have done the same in hindsight tbh.