If my parents use the land registry to add me as a 10% owner of their house, would I still qualify as a first time buyer in reference to common first time buyer mortgage schemes?
In the UK this would likely disqualify you from most "first time buyer" schemes.
For example, the Ts & Cs for the Help To Buy ISA specifically define:
“First Time Buyer” means an individual who:
(a) is not, and never has been, a Residential Property Owner; and [...]
“Residential Property Owner” means an individual who owns an interest in land, whether in the United Kingdom or elsewhere [...]
However, ultimately you would need to check the specific definitions that an individual lender or service provider is using.