Jipe's info is only half correct. Brompton announced with the move to the new factory their goal to double sales to around the 100.000 a year mark until the early 2020ies. It seems unlikely that they will reach that goal or even come close, based on the published sales numbers. Sales stalled indeed for a couple of years (after years of massive growth) but they started growing slightly again last year as you can see from their published financial data. Brompton's business year lasts from April-March. These are the numbers I drew from their official publications:


So you can see a slight growth, especially in April 2017 to March 2018. What's more important is that for the calendar year 2018 the frame numbers of produed bikes indicate that they have probably built more than 50.000 bikes during the calendar year 2018 and will thus possibly end up beyond the 50k mark for their business year 2018/2019.

This is IMHO however not really a massive leading indicator for the folding market as Brompton expands into new markets and enhances sales through more intensive marketing than they did earlier as well as new/different sales channels. Still I do not see a fall in folding bikes - the opposite is the case. The number of folders I see in the wild has grown massively over the last years. It is still a small niche market but my impression is that is has been growing and is growing further.