At first glance forgery seems like a very real threat considering how hard it is to tell vaccine from sodium chloride (entire testing procedures are based on this), but the same difficulty is also working against a black market: why pay if the seller can't give event the slightest indication that it's not a fake and the fake has no intrinsic value at all? You'd have to fake the distribution structures and not the product and an elaborate fake structure isn't something that suddenly pops up from some dark market investment, it could only evolve from simpler black market schemes. But those won't happen, at least not in time (except maybe in places with a truly corrupt regular distribution system, where it would start with "redistributed" real vaccine and then slowly shift over to fakes)

The UK government has ruled out having a vacination passport (for good reason in my opinion).

Strong opinions, loosely held.

The UK government changed their mind about a second lockdown given new evidence. The virus got much worse and they responded. What new evidence is there that the UK government is going to start vaccination passports? Has something changed?

That's the wrong question. The right one is "may something change in the future?" And an answer on this: "maybe, we don't know, nobody knows the future".

That being said, extrapolation of the exponential curve a month forward is one of relatively simple ways to predict the future. And yet, it was _completely_ unexpected for the UK government. Think about that.