Sharing my method for removing the rear triangle.
Once you remove the first bolt, which is usually easy, you stick a second allen key (5mm tends to work) in the axle and put as much force/tension as you can on the allen key to prevent the axle inside from spinning when you loosen the other bolt. Using a breaker bar or an extra set of pliers to clamp onto the allen key will help get more leverage.
I've used this method on several frames and its worked without the need for heat.
PS I can't take credit for this method, I saw it in a youtube video or forum once but I couldn't find the post again so I thought I'd make my own video about it.
PPS I also shared this on the FB Brompton Hacks groups which is a great resource but the content doesn't really show up on searches so hopefully people can find this post if they're looking for an easier solution.
by edtse88