Well yes, investing your money carries certain risks which you need to understand and be comfortable with- but with investment funds you can set your level of risk according to your circumstances. Very low risk portfolios won't make as much interest of course but then your money is safe. Investing in funds linked to housing markets wouldn't be in that very low risk bracket for sure, but what you have to remember is that in buying the BTL you are investing capital in the housing market just the same as if you bought stocks linked to the housing market, with the additional issue that you are incurring a large liability/debt in the form of a mortgage. So in a nightmare scenario e.g. a 50% drop in the housing market, if you've bought a BTL you lose 50% of your capital, probably lose a chunk of your rental income as well as tenants will expect lower rent, but you still have to pay the mortgage and can't liquidise the asset or move funds around easily if you now have very low equity or even negative equity. Normally the housing market will recover even from significant drops but it can take years depending on the local area. Whereas if you'd invested it, yes you'd lose 50% of the capital but in theory your new purchase would also be 50% cheaper and you would still have the remaining cash available to easily access and use for the purchase. That's a simplification of course but my point is don't make the assumption that bricks and mortar are automatically safer than other investment methods, in the short/medium term in particular.
Landlord insurance can cover lost rent yes, but it will cost you more. That £15 pcm I quoted was for basic cover only, not accidental damage, legal cover, rent guarantee or furniture and fixtures cover. You can also get rent guarantee schemes (where effectively you rent your flat to a third party such as a housing association or commercial provider) and they then guarantee your rent regardless of void periods etc, and they take responsibility for renting it out - but this isn't always allowed by mortgage providers if you wanted to go down that route, you'd need to check it out and perhaps speak to a broker.
Also, re your Help to Buy ISA, I don't think you can cash this in to buy a property you intend to rent out, even if you will initially live in it, check out this article.... blog.moneysavingexpert.com/2016/04/can-you-rent-out-a-home-bought-with-a-help-to-buy-isalifetime-isa/
Sorry to sound negative and of course there are benefits in owning a property that's all yours, for instance I know some people take a lot of comfort in knowing there's a property in their sole name if they are not married to their OH, in case of a relationship breakdown or similar. Equally if you think you might want to move back to that region at some point I can see the point in keeping a foothold in the market. But there are other ways you can protect yourself and for me personally I'd rather invest my money in getting as good a property to eventually live in as I can/as small a mortgage as possible, whilst retaining a reasonable amount of cash not in the housing market at all (as a safeguard against market crashes), rather than having all my capital tied up in a BTL and stretching myself both practically and financially to maintain 2 properties...
If you are very new to all this sounds like some financial and investment advice wouldn't go amiss, any accountants or independent financial advisors amongst your family/friends who could point you in the direction of some reputable and unbiased advice (you may have to pay but worth it IMO)?