Most ATX PSUs have a 5V standby, negating the need for any fans to be running. The 300W unit I sometimes use as a bench PSU, has a 5V 2A standby output, more than enough for a PI.
For a different battery option, LiFePO4 maybe worth a look. They are more robust than LiPo batteries, less likely to catch fire and don't mind a bit of abuse. They weigh slightly more than a LiPo and can't deliver as much current, but for a 700mA PI, that shouldn't be a huge concern.
Hyperion have a range of LiFe packs for RC receivers and transmitters: http://media.hyperion.hk/dn/fg3radio/
Finally, I noticed earlier B&Q have some 1A USB plugs at the moment for £3.98, which seems pretty reasonable to me and would hopefully be a good quality unit.