Looking for a 'Dumb' USB Battery

Why not just build your own battery pack without all the tech $hit. USB cords and multi-cell battery cases are cheap enough. That way you can build it to what ever capacity you want or need.

https://m.youtube.com/watch?v=iz-bu5aWZNc

Look on eBay

Here is a search I performed for "mini usb battery pack"

"... I want it to maintain the voltage until its battery is empty...", and therein lies the rub- "empty" is defined as the USB terminal voltage being below 4.75 V or 4.45 V for USB 3.0. The USB/battery power controller inside your GPS tells your device's power management system when the external voltage falls below that level and then switches to the internal battery. You can't fool it to accept a low USB voltage, it will shutdown the device if the internal battery doesn't have the required voltage.

Yep, that's fine :-) as long as it can hold the requisite voltage for a few days against a device that is only gently sipping sparks (the inernal 1300ma/h battery lasts a week under normal usage) then I'm good with that.

I use a variety of these E-power and Panther power battery packs for all my devices when I go off grid....Just look for one that's compatible with your device...

You could complement your battery back with something like this:-

http://www.solartown.com/learning/solar-outdoors/guide-for-selecting-solar-backpacks-and-bags-taking-civilization-into-the-wilderness/

to make the best of both world.

On the otherhand it might not be a good idea, as I believe the weather in Wales isn't conducive with solar items. ;-)

You need to figure out how much average current your GPS uses over say an hour. Then multiply that time the number of hours you need it for. That will be your battery capacity in amp hours. Then take your GPS battery voltage and divide it by 87.5%. That will be the voltage required to maintain your GPS for the time period you need. For example, if your GPS has a 5V battery that uses 100 milliamps per hour and your trip is 72 hours then you will need a 6V 7Ah battery. Then make a constant current controller ( a regulator and a few resistors,) attach a usb cable to it you're good.