You need 66k salary to 'buy', flat costs £190k, is 'worth' £600k, rent is 900 a month.
Downsides I can think of:
I don't believe it's 'worth' 600k on the open market, they're not really being sold at that without the shared ownership part. Seems about £120k overpriced for the area by square footage.
You will never pay off the rent, will still be paying it in retirement unless you can sell your portion and then move - that's a lot of rent to go down the drain.
Salaries are unlikely to go past the £70k mark as well established public sector jobs at top of pay band already - so no opportunity to buy a greater portion (who could afford it with rent/council tax coming in at over 1200 a month)