It sounds like you’re suggesting that there is a causal relationship, with the increase in BTL being the cause of fewer people getting on the housing ladder.

Another explanation that would fit the pattern just as well could be that the causality works the other way round. Maybe fewer people are getting on the housing ladder for other reasons, so they rent instead, which increases the demand for rented housing, thus creating a buoyant rental market which encourages more landlords into the BTL market.

Do you have any data that might help distinguish between those possibilities?