Well, the limits are set at the level of each bank's overall PDH book in any given year rather than being set at the level of each individual applicant. In other words, not more than 20% of the value of all PDH loans written by a particular bank in any year can exceed the 3.5 LTI ratio.
It might be worth talking to loan officers in a few different banks given your target LTV but, to be honest, I wouldn't be very optimistic about your chances. An LTI ratio of 4.2 is really very high unless you have an unusually large income. Are you sure you can really afford this move at the moment?
I certainly take your point about some of the online mortgage calculators. I'm surprised the Central Bank hasn't clamped down on this TBH.