You're missing out on other aspects of hiring like recruiter fees. If I were to hire every idiot that ticked off the boxes but couldn't demonstrate relate knowledge to something tangible I would've put my current and previous company out of business.
Your 90-day long interview process (this is what it is, face it) is not only a financial detriment, but a productivity tumor. Yes, I do agree that the current wide-spread use of "solve this on a whiteboard", "implement merge-sort", or some other backwards method of technical ability does not work; that is why I implement questions, and paired exercises that are tailored to the position they will be filling and the background they're coming from. The process I have in place consists of 3-parts: phone screen, and 2 in-person interviews - this process normally doesn't extend past 3 hours. The interviews are structured as a conversation to make sure the most nervous of interviewees can perform comfortably. I'd rather waste 2 hours more per candidate opposed to ~20-40k GBP for a poor trial, and a decrease in overall team velocity.