UPS recommendations?

Because they want to make money and there's no market for such a thing.

I've not really been following this thread so please excuse if this has already been suggested/discussed.

Something like this

Shopping list:

Plus maybe (sticking with simple, electronics people will likely cringe) something like

.. and were it just for the RPi (which yours isn't, I appreciate), you might like a large battery which can be float-charged at the same time as powering the RPi. For example, the 10400 mAh unit here, stock number

er

Where does it say it can be charged at the same time as being under load?

Oh and Sainsburys (at least Penrith) have the TP-Link 10.4 AHr Power

Amazon.

The TP-Llink also doesn't say it can be charged and used at the same time. Also any supply needs to supply the charge and load current so lets say 4 A. The TP-Link is up to 2 A on it's input, the RS doesn't state a max input current.

And the maximum draw of 2 A and 1 A per port on the TP-Link or "Maximum Power Supply is 2.1 A" on the RS, I'd assume that is 2.1 A from only one port or a tad over 1 A from both. Might not be enough umph, remember there is a DC-DC convertor in there somewhere to boost the nominal Li-Ion 3.7 V to 5 V.

I've actually used the RS Components one, and confirm that it can be charged whilst under load. That's why I recommended it. The input is rated at 1A, so the maximum continuous load for full charge would be one RPi.

If you get one of the TP-Link devices it would be interesting to hear your report.

BTW: I got one of those USB voltage and current monitors - cheap and gives an indication at least!

I'd been looking at one of these:

Just be sure you can charge the battery at the same time as using the USB 5V outputs.