@Milajah I'm not a lecturer?
The benefit of property investment to the economy is not singular: landlords not only have greater taxation dues (£16.1bn a year), but they also tend to have greater expenditure in the economy. This contributes through VAT, but also by boosting revenue in local economies - specifically relevant to the typical " 'ave a go amateur Landlording" you claim as your example - who aren't keeping this income abroad and out of reach of HMRC. These injections into the economy have a significantly higher multiplier effect that provides substantive benefits for all through recirculation. The extent of these depends on the type of expenditure, so won't be able to provide a statistic here. With a housing crash would come a credit crunch too, and that isn't something that would benefit anyone. I don't want anyone to struggle through a full and rapid enforced restructuring of the economy, and hope I'm not alone in that.
The direction of causality argument is valid, but doesn't help your position. Because removing or significantly limiting BTL in the economy would not boost housing markets, but would instead likely bring on a structural recession, this would mean that economic uncertainty, a broken housing market, and all of the other joys associated with downturns would occur regardless of the route of the causal relationship. (And as to the direction, it's likely reciprocal)
What I'm saying is that placing a significantly greater tax burden on BTL landlords would not generate any benefit for existing tenants, while also damaging the economy significantly. This is unequivocally a negative for our country, with international ramifications. If you genuinely want the result of people being able to afford deposits, it's likely that mortgage industry regulation is the appropriate avenue. If you want a more diverse economy, I don't disagree at all - but that has nothing to do with BTL, and the suggested tax would not generate this result and may even do the opposite.