Hello Dewy,
I am not sure what is available in England however in the US battery powered air pumps are usually easy to find. If what you are keeping does not have gills then a short power outage will most likely do no harm, although you may have to do some partial water changes if the outage is extensive. In terms of a potential power outage, I am worrying more about the AC going out during one of our eastern US summer heat waves, given that for the time being my animals will no longer be housed in a basement.
Chip
