The brakes on the 2000-ish L3 I had were absolutely fine with new cables.
For the record the brake history on the Brompton from memory:
Starting with the MK2 at the end of the eighties Brompton were using brakes by CLB. Those can be identified by a black piece of plastic on the front side of the brake axle (and look pretty similar to those used on Moulton AMs of the 80ies). Somewhere in the early nineties they switched to Sachs single pivots for a couple of years and after that to Saccon City calipers. All those brakes leave something (or a lot) to be desired in terms of brake performance. However: Better brake pads, better levers and better cables can improve a lot.
With the MK3 at the beginning of 2000 Brompton invented the dual pivot brake made by Alhonga in the front, at that time in silver. On the rear there was a new single pivot brake, probably from Alhonga as well. These brakes are fine, braking power is absolutely sufficient. This dual pivot was factory standard until the new brakes were invented at the beginning of 2018. Only the color changed over time from silver in the beginning over chapmpagne in about 2008 to dark grey in about 2013 (and black on the black adition models from 2015 on). The rear brake changed from single to dual pivot in 2007, second half I assume. No doubts about break performance with this one as well. Still the single pivot on the rear from 2000 on can be considered to be sufficient as most break power goes via the front brake anyway and the bake is better than it's anchestors.
2018 brought the actual brakes, in all black on all models. They are again dual pivots but of a different design. Front and rear brake are now identical and exchangable, they do only fit on older bikes after drilling the mounting hole on the front fork (and the front blade) a little big bigger as they use a different type of nut. I could not recognize any difference in brake performance between the actual brakes and the former dual pivots.
Brake handles/levers:
The first couple of 100 (or maybe 1000) bikes had an edgy metal brake lever, rarely seen on used bikes, as it was used only on very early ones. After a short while they got exchanged for the slim sticky looking ones that are common on Bromptons. They are lacking the possiblity for adjustment of lever or cable.
With the MK3 there came a newer generation that offered cable adjustment, those are called "open clovis" due to the way the cable is mounted inside the lever and had a, I think, 30° angle.
2008 brought a redesign called "closed clovis", again due to the way the cable is mounted inside. It offered not only cable adjustment but also lever adjustment (not sure, maybe the generation before offered it as well) and offered a, I assume, 15° angle. Brompton says the open clovis design levers should be exchanged as they do no longer cables for it and with the wrong cable it may slip out of the lever/clovis (and the rider out of brake power).
2013 brought the modern, edgy brake levers that look a bit like Avid ones (but are a Brompton only design) and have a silver corpus along with black levers (exactly opposite than all generations before). They definitively increase brake power and handling massively to the better and are from my point of view highly recommended for any generation of brakes.
2017 brought a revision of these, they now offer mounting holes for the new gear shifters that were invented in 2017. Again, from 2015 on the levers are completely black with black edition models.
Other than that: Since about 2008 Brompton used Fibrax brake pads that enhanced the braking power and the rubber was exchangable in the carrier for any Dura Ace type brake pad. Jagwire compression free cables were invented with the S model, possibly as early as with it's invention in 2005, maybe a little later. Over time the other models got those, too.
So with any Brompton I personally would switch to the newer style levers and to better cables and brake pads as this will increase the brake power massively. With any MK2 I would seriously think about upgrading the calipers - it is expensive but again a massive boost in brake power. A fair amount of MK2s have been retrofitted already in the early 2000s as Brompton offered a retrofit kit - possibly knowing how bad their brakes were before the dual pivots.