Helix Ultralight with 20.3LBS naked is significantly heavier than the 4s T-line 7.95kg that include mudguards, front block and front+rear lights.

If you upgrade the brakes, you will also save some more weight

And its bigger folded than the T-line.

For the transmission, according to information found on the web, its easy to add one cog to have a 5s and a (slightly) wider gear inch range.

Now, I agree with you, instead of spending money to develop and manufacture their ridiculous 4s proprietary derailleur, Brompton should have taken a look at the transmission of the Birdy that has 11s and a nice wide range of gear inches with a standard reliable and not expensive at all Shimano 105 11s (or the excellent 11s Ultegra) and a reliable chain tensionner around its bottom bracket.
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