Continuously rated, the Johnson CM30P7-1 pump runs on 12V (just plug into your PSU), 2.2A, 26W; pumps 356 GPH / 22.5 l/min at 3ft. head.
Dimensions are 18cm long by 5.5 cm width.
The only problem is the price... $179,-- i.e. £130,--. Ouch! And it gets a bit warm: 53C, but it is still possible to put it in a case without adverse effect on case temps.
The big advantage is that it is tiny, and simply plugs into a molex.
Whaddaya think?