Brompton owners

I had the same question a couple of months back! FWIW ended up with the latest P line secondhand (third but very close to new condition tbh) for 30% discount. Which seemed like a ‘good price’ as I wanted 4 gears, and other recent improvements. I didn’t want to take on a whole project. The P line is lovely (still miss my Ni edition though, probably best finish for steel framed Brompton) but there is plenty room for improvements (grips, rear triangle protector, saddle, seatpost, pedals, etc) if you want to tweak things

Pondering selling mine, singlespeed.

I think the P-Line plus mods is the way most people get the weight down.

depends on how much you want to spend, and how quickly you want to lighten the load. Plenty on here have lightened their bromptons, myself included, but it gets to the point where the cost starts to outweigh the benefits.
think your biggest weight saving gains will be seat post, rear triangle, wheels, pedals.
I brought my 2spd on cycle 2 work scheme for about £1100, think actual cost to me was like £700, 7 odd years ago, and have spent approx another £700-£900 swapping out seatpost, rear triangle, handlebars, pedals, saddle, crankset, front wheel, and other little bits to get it to how I'd like. So it could be lighter, but I've got mudguards and a dynamo front wheel, because it's a use in any condition town bike..
Personally think you're best bet is to try and get a chpt3 or similar ti lightened brompton secondhand and modify as you'd like..

edit for clarification.

P for pricey. Would still consider one on CycleScheme or similar. I'm no weight weenie but do like a good saving, and try to avoid putting on heavier components if I can help it. Here's some changes that I made to my Brompton that shed some weight to the hefty, micro beast.

• Swap Stock tyres - I had the standard wire-bead Brompton Kevlar (420g each?) and now run Conti Urban Contacts (folding, 275g) and sometimes wire-beaded Schwalble Kojak's (220-ish g). More for speed and ride feel.

• Super light front wheel - Only did this swap after running my stock wheels for many years. No massive weight savings, but still something and it makes the ride feel nicer.

• Saddle - The Brompton Saddle never agreed with me, to the point I went for a much heavier Brooks B17. More recently I'm running a much lighter (and cheaper) Charge Spoon.

• Pedals - I know some people love the folding pedals and the small profile it lends to, but I don't. Poor grip and a small platform for many shoes/feet. I swapped my pedals not for weight, but saved a bit along the way. There's potential here.

Then there are some areas I added weight on. Changing things like the rear triangle, seatpost etc. can get a lot more costly, which is why I've steered clear.