I am saying it should be variable. So, as per your example, if the limit is £100,000. Someone on £99,999 should only get a 50p bonus if they save 50p (obviously those numbers are ludicrous but you get my drift)
Variable is a nice idea, but then you need complicated means testing calculations. See tax credits/universal credit - nice idea in theory that there isn't a cliff edge and the benefit tapers down so is mostly concentrated on those with the least money and goes down gradually to nothing as earnings increase, but then it seems to turn into a whole mess of mistakes and overpayments that need to be clawed back or weeks and weeks with no money until it gets sorted out.
It shouldn't be beyond the wit of (wo)man to knock up a few spreadsheet functions to calculate entitlements based on what appears to be a fairly simple formula/set of rules, but endless complaints on here, in the news etc prove the opposite.