There are a few things to unpick here.
My belief that Brexit will lead to lower NHS funding is as follows: assuming Brexit results in higher trade barriers with the EU, our biggest trading partners, it will dent trade, which will lead to lower growth, which will lead to lower tax receipts, which will lead to lower funding available for the NHS (and other things). The unknowns are by how much trade with the EU is affected and how quickly trade with other countries grows to fill the gap. Given current trading patterns [8] I feel justified in thinking it'll take some time. The other points are basic macroeconomic principles.
Leaving aside the factual points addressed below, your belief is based entirely on how you think Brussels thinks, how the UK elite think and how they will respond to some hypothetical event. It's basically playing out a possible sequence of future political events. I would be interested in links to the informed analyses you mention (I'm not sure what search terms you think I should use).
What's interesting about it is that it's not falsifiable. It's your political opinions, which can't be proven either way, and playing out a hypothetical, which is unknowable. My belief can be disproven easily by demonstrating how Brexit won't harm overall trade or some plausible offset that I haven't considered. Your belief is correct if your reading of the entire EU and UK political classes is correct and you correctly predict their intentions. My belief is correct if we have a correct understanding of how trade affects growth. Do you think that both beliefs are equally robust?
On factual points, recent data [1, 2, 3] shows that the UK is actually underperforming the Eurozone in growth, including Germany (though it's also experiencing negative growth). In addition, this is from a bad start as the UK has recovered very poorly from the GFC [4], especially relative to other major economies [5]. On your final point it is at best highly questionable [6, 7] that fewer EU migrants will lead to higher funding for the NHS.
The fact that this is considered "obviously true" in the face of evidence to the contrary is a very good example of what I'm talking about. I agree we should challenge our own views. In particular, I worry that the argument that EU migrants are a net benefit overall is all well and good on paper but ignores the localised impact in places that experience high immigration and don't see a corresponding increase in central government funding. That said, given the evidence of the relative economic consequences of austerity and EU migration I feel justified in saying that people "should" direct their anger at the government that mishandles migration, not migrants themselves.
[1] https://tradingeconomics.com/european-union/gdp-annual-growt...
[2] https://ec.europa.eu/eurostat/statistics-explained/index.php...
[3] https://data.worldbank.org/indicator/NY.GDP.MKTP.KD.ZG?locat...
[4] https://www.ifs.org.uk/publications/13302
[5] https://blog.euromonitor.com/the-recovery-from-the-global-fi...
[6] https://fullfact.org/europe/eu-immigration-and-pressure-nhs/
[7] https://www.niesr.ac.uk/blog/migration-and-public-finances-l...
[8] https://www.ons.gov.uk/businessindustryandtrade/internationa...
