I'm looking at buying the pro trinket and the liion/poly add on. It says not to use it with any battery except the adafruit battery, but you don't seem to be able to buy that in the UK. Does anyone know something else I can use?
I'm pretty sure you can use the batteries sold by Pimoroni: https://shop.pimoroni.com/products/lipo-battery-packI'm using a Adafruit Huzzah powered by very similar batteries I bought from a competitor (ssshhhhh!). I'm even charging the batteries with an Adafruit USB charger.
The pain with those batteries in UK is you can't use Royal Mail.
All the best!Jason.