Power Bank with iSmart 2.0 and other features for £20.29 (down from £28.99, by Camelcamelcamel usually costs £30)
Product Dimensions: 6.8 x 12 x 2.3 cm ; 381 g
They also offer 26.8Ah
variant for £21.40 (down from £35.99), but that one seems to be too big and heavy - 17.2 x 8 x 2 cm ; 458 g
I bought it with a car charger
and received it (the car charger) with 50% off promotion (just click on "add both to basket")
Both have good reviews and are Fulfilled by Amazon.