The device that looks like a UK 13A plug with a usb socket has an output of nominally 5V dc (usually 5.2V) with a capability of providing various amounts of oomph, 500mA, 1A, 2A. If you choose a 2A version it will do for all applications.
For comparison a standard computer usb socket can provide 500mA. Some equipment like hard drives or raspberries need 1A and have to be connected to two usb sockets in parallel. Er, not a happy arrangement.
Be sure to unplug the usb cable before turning the mains off. In some circumstances the unpowered charger can short the device battery. I had this happen to a Kindle and had some difficulty getting it going again. Shouldn't be possible but it did happen.
USB sockets in cars again should all be 5V. Some devices come with chargers that plug into the cigar lighter socket, again these should produce 5V dc. Be sure that your device is isolated from the rest of the car, and not plugged into anything else.