I actually wrote something and forgot to post it yesterday, but this is how I feel on it:
I think the vast majority of folks trying to install Espruino on F18s are just excited and want to get started sooner, and that's great!
However I do need to focus my time on getting Bangle.js out to Kickstarter backers and those that preordered, so I'm not going to be able to provide masses of support.
Doing full wireless firmware updates is a bit of an elongated process which requires a bootloader to be dumped off the device via some very hacky means and then binary patched. You're looking at doing 4 different firmware uploads. If any of that goes wrong (which it may) you'll brick your watch and have to open it and use an SWD programmer. I'd be pretty certain that DT No.1 won't like having bits of their watches firmware posted online and I want to stay on their good side so I'd ask that nobody else does either in case they refuse to let me buy their watches.
If you want to compile your own firmware from source, open your watch, and flash via SWD as @sebi has done then that's awesome! Although if you're thinking of doing it, bear in mind that the cost of an nRF52DK and F18 watch may be more than the cost of just buying a Bangle.js so you'll be doing it for fun rather than to get a bargain :)
Please don't try and hide that it's a DIY Bangle.js if something is wrong though as it may actually make it more likely I can help to solve your problems, as has happened here.
Also, if you've opened up your watch and flashed firmware via SWD you're in a great position to do some development work on Bangle.js firmware, so it'd be a massive help if you felt like wading in and suggesting some tweaks/changes :)