If it helps anybody, here are step-by-step photos of removing the foam grips from my Brompton and replacing them with the Ergon GP1-L Biokork grips. Total time about 45 minutes if you're fussy.
Note: Check that the clearance of the handles on either side (distance brake and/or shifter attachments re the same distance from the bar ends). I found my left ones to be 10 mm different from the right side, meaning I would have ended up with two differently measured and cut Ergon grips if I wasn't paying attention, unless that's what you want, of course...
I debated removing the foam grips in one piece based on another blogger's write up that wanted to keep the grips, but I decided it wasn't worth my time. I used a utility knife and cut at an angle through them. I didn't cut knife point down to avoid scratching the handle itself. I did keep the cut grips though figuring it might be useful for padding.
After cutting, you can "peel off" most of the grip and slip it off, except for wherever the glue is. Note to Brompton: such a nice bike, and you glue foam handles on? Seriously?
To remove the residual glue/foam bits, I didn't want to scratch the surface, but obviously just using my hands to scrape the residue wasn't going to work. I found a used nylon dish scrub pad with some lens cleaner (dish detergent or rubbing alcohol would probably work well too) worked well. You may have some glue gunked into the hollow bar ends - more about this later.
You obviously want the new grip to be fully inserted on the bar, which means you need 130 mm of clearance according to Ergon's instructions. I found that my Brompton only offered 102 mm of clearance from bar end to shifter/brake attachment. I felt that if I had cut the Ergon grips to 102 mm length, I would not have enough hand surface remaining to comfortably and securely hold the grip. I noticed the inner plastic sleeve of the Ergon grip is in two pieces: about 2/3 of the length is towards the narrow end, and 1/3 of the end is towards the wider, locking end. I think only the outer 1/3 portion that tightens around the bar as the locking bolt is tightened. If so, I figured if I extended my grips a bit to about 112 mm to 114 mm total length to give me more hand surface, enough of the outer sleeve will still lock around the bar, and the force of the bolt itself is still reasonably inline with the handle to not fracture the locking sleeve under compression. Yes, I realize this might not be the safest, but I figured it was the best tradeoff. I can also still cut more off in the future if necessary. Note: I can see the extra length being an issue if you have and want to use stems on the grip: probably would create too much unsupported moment.
I used a hacksaw to cut because I wanted to avoid a jagged edge as much as possible. Since the grips are irregularly shaped, I used a vice to hold them.
The cut came out pretty clean! Some sandpaper took off any rough edges. Hint: use your finger to support the end being cut to prevent too much movement and cut with less force towards the end to avoid breaking the material as it weakens with each successive cut.
Here is one grip mounted. Looks sharp!
This is what I meant by available hand surface. Even when cut to about 113 mm, I don't have much left to grasp onto. 102 mm would have been tough to hold securely for me, and I don't have particularly large hands.
As a double check before I pounded in the grip plugs, I wanted to make sure the grips cleared the floor since I left a bit extra length. (They do!)
Here's the odd thing about the grip plugs. I wasn't able to get all the glue out of the hollow bar ends, and so they impeded getting the plugs in without significant pounding (rubber mallet). However, they would act somewhat as compression supports when tightening the locking bolts, so I wasn't overly concerned about damaging the grips (don't overtighten obviously).
All done! They feel great too when riding.
That's it! Next mod.. maybe some roller wheels for the rack. Hope this was helpful to those interested in installing the Ergon grips.