Its reckoned we’ve lost 2.5% GDP so far. I don’t know if you’d call that a severe economic downturn. Also, some of the current GDP has been generated by preparations for Brexit, but I’ve never seen any figures on that.
Other things like the loss of R&D expenditure is only going to reveal itself over time, and probably wont be very trackable even then. Also there is surely a slow process of contracts running out and not being renewed.
When and if we do leave we know a lot of companies have opened ‘forward operating bases’ in EU countries with basically skeleton staff at the moment. If Brexit finally happens we have to assume more jobs will transfer abroad - and they’re all higher end jobs too.
I doubt we’ll ever know anything other than estimates. E.g. Losses to R&D expenditure may be relatively small in the overall scheme, but potentially huge if companies makes significant breakthroughs that would have otherwise been based in the UK. It is hard to put value on that, just one could lead to a whole series of new patents that might even transform a whole sector.
What doesn’t seem to be in dispute anymore, from any side, is that the UK has lost out through this process, and in a big way, and that the losses are going to keep adding up into the future, whatever the outcome. Meanwhile, the supposed benefits have added nothing so far of real worth (a lot of largely wasted expenditure in preparation) and that side of the ledger has a long, long way to go to even start to catch up.