Although that 200k increase looks good on paper, often in reality it means that the property you want to buy has increased by £300k.
But that's just assuming you should just be able to buy another more expensive property based solely on the fact your property has increased in value, rather than the fact you've earned more money elsewhere or are able to get a bigger mortgage.
It's not just a problem when upgrading. It would be an issue if you wanted to move to another part of the country and buy a similarly-valued house. It would cause problems for household formation when each party owned a property beforehand.
But that's just assuming you should just be able to buy another more expensive property based solely on the fact your property has increased in value, rather than the fact you've earned more money elsewhere or are able to get a bigger mortgage.