With many dealers you get a discount for bikes already in the shop during winter season (~Nov-March), often around 10%. If you are outside of the EU I would not expect a massive change regarding list or regular market prices through Brexit. Brexit doesn't change anything in terms of taxes or duties for customers outside the EU plus Brompton have created local distribution daughter companies i.e. in the US, in Hongkong (for Asia) and in Benelux. Those may be able to stock bikes in advance to Brexit and will buffer price changes and currency risks or changes to a degree longterm. So short hand I would not expect relevant changes for end customers. Other than that the Brompton factory seems to run at it's maximum pace currently - at least here in Germany stock of the distribution is close to non-existent and custom orders take many weeks to be fulfilled. So if you want a new Brompton you either wait for one that is built custom for you or you take from the very limited selection of what's instantly available. Has not been like that for many years. no reason for dealers to discount bikes other than older models.

As far as foreseable there will be no massive technical changes for next year, so no massive rebates for soon out-of-date-bikes to be expected. As prices are likelier to rise than to fall as Brompton tends to rise the price each and every year a little bit I'd assume it is not worth waiting for a speculatively cheaper price other than a possible off season one. The latter may be of interest if you are not already bought into a certain combination of model and color but will still be a bet.

In the EU Brompton prices may rise a bit after Brexit but that's again a bet and nobody knows what will really be happening.