Isn't it that different components have different warranties? I was unfortunate enough to have one with the dodgy BB, that wasn't under warranty, got replaced by non-brompton certified place that had a huge Brommie fan mechanic (because I didn't want to buy two BB tools), at reasonable price (because store where I'd bought were quoting ridiculous prices), about a month before they recalled :-(. Have I further reduced warranty by getting my local cool shop to do the repair for me?
However, when it comes to some mods, Brompton could certainly claim that if something has been modified that might have an influence on some other warrantied item failing early, or simply that it hadn't been properly serviced by a Brompton authorised repairer. Certainly, many parts, they wont supply to non-authorised repairers, preventing smaller stores from buying in parts to do certain repairs, especially around forks, headset, etc.
When it comes to gearing, changing from the standard 50T chainring on the 6-speed, to a 44T is an easy change to make that wont affect any part of your warranty, and just requires a 6mm allen key. When you're in the city, you might want to put the 54T on instead. You might want to remove a couple of chain links going from 50T to 44T, but if you're just trying it out, it's not necessary. I rode my 6-speed from London to Cardiff last year with a 54T, with a 44T as a backup in the bag. Once I got there, fitted the 44T because I knew I was going to do some sensible climbs in South Wales, but without master link pliers, could not get the link to split. In the end, just kept the chain at length for 54T, and while a little too low, it still worked fine for the next couple of days of riding 200km about about 2000m elevation. I don't recommend this long-term, but tensioner will take up the slack difference between the two chainrings. But really, use right chain length. If you shorten, and then want to put back if you want bigger chainring for city use, then two quicklinks is fine too, to add back the bit you removed.
