After many happy years with an Alpkit Possum frame bag, I replaced it a year ago with the Restrap equivalent. Can’t comment on the custom options, but the stock Restrap bag is currently quite a bit cheaper than the Possum (on Merlin).
The Alpkit bag, bought 2014, lived on my bike for seven years. The zip eventually failed due to me stuffing too much in. The Restrap bag is nice, feels like a quality item; maybe needs some padding along the surface that lies against the downtube to stop stuff inside clonking against it (never a problem with the Possum, or the Wildcat Ocelot that lives on another bike).
It’s all UK made kit, though, and does the job. Just packed for a couple of days away starting out of the door in the morning.