This isn't totally true. The UK was an attractive place to make films before (due to a combination of very good facilities, tax breaks, it being located near Europe's VFX HQ and the fact that dailies can be done in time for Hollywood to wake up) and it's even more the case now when it comes to foreign investment. Half the Marvel films and all the Star Wars films are filmed at Pinewood, just north of London, and they're being produced by a company which literally owns its own film studio. NB, I work for a film company though we mostly do advertising, and don't do any feature film work. Domestically financed films that require an international audience to justify financing are going to be in a bad place re: being produced in the UK. But that's a pretty small market. Most feature films produced here are funded abroad, where actually the drop in value of sterling makes it more attractive to get your film made here. Mrs Brown's Boys is obviously in that former group, unfortunately for them.
Going to start of with two minor (and let me stress that they're minor) things:
Mrs Brown's Boys did not rely on an international market to justify its budget. But was produced in Ireland and shooting in Ireland is costlier post Brexit, hence the postponement.
To the best of my recollection most feature films aren't financed from outside UK, but they do account for the majority of the majority of the money spent in the UK film industry.
The blog linked to earlier has two really terrific articles on the Film industry and Brexit:
How will Brexit affect the UK film industry?
What does a post-Brexit UK film industry look like?
What I didn't talk about in my previous post in detail was the end of EU funding through MEDIA, and from the first link above:
An end to MEDIA / Creative Europe funding (certain). Between 2007-13, the MEDIA scheme provided over 100 million towards various aspects of the UK film industry. The loss of this money is the biggest, clearest effect of Brexit and so Ive addressed it in the section below.
The blog also talk about outside investments:
It becomes cheaper to shoot in the UK (uncertain). If the Pound continues to lose value (as it has since the Brexit voted was announced) then the UK becomes ever-cheaper for foreign productions to set up shop in the UK. This will have the biggest effect on Hollywood studios, who spend vast sums of money and whose green-screen epics can be shot almost anywhere in the world. Between 2006-15, UK / USA studios films spent £7.7 billion in the UK, accounting for 68% of the money spent on UK films.
I don't really have anything to add to the articles linked, they talk about tax breaks, how the EU finances UK film and much, much more. And they do it in a better way than I can manage. But overall I can not see how Brexit is beneficial to the film industry at large, and neither does most of the people asked in the survey from the second article linked.