I would go with a multi-channel amplifier.
A 12 channel amplifier from any of the major manufactures - Niles, Speakercraft, or Sonance. Check eBay.
Each of these amps can power up to 24 speakers.
Each of these amps have individual gain (volume) control for each of the 12 channels of output. The volume controls are typically on the back of the units, but are usually easy enough to get to and adjust. If you end up not happy with that operation, you can add a separate volume control(s) before or after the amplifier.
You can go direct into the amp from a PC or iPod if it has internal volume control. A PC does and a iPod does - a stand alone CD player does not, but you can use the CD drive inside a PC/laptop for CD playback just fine.
Something like this is ideal:
SpeakerCraft BB1235 12 Channel Big Bang Power Amplifier 1940 | eBay
Another option is to put multiple volume controls throughout the space in different areas and allow people to adjust the volume locally for each pair of speakers in their space so that they can choose a bit more volume level if they would like it.
I actually did a fitness center recently and this is pretty much how it was setup...
Several sources were available through a mixer which also allowed for the receptionist to use the microphone to speak through the speakers. Three or four different sources were available to the system. Two separate stand alone systems were on a cart for their workout rooms which were separately enclosed rooms.
They also had local TV feeds with headphone connections to a bunch of stand alone workout pieces of equipment with dedicated TV tuners for all of them. A couple dozen maybe.
Obviously, this WAS tens of thousands of dollars to setup, but was part of a hundred milliion dollar plus building.
You really do want to have more speakers if possible though so that you can have even audio coverage throughout the space.
Unfortunately in the UK, you don't seem to have the same number of audio options via eBay, which may throw things off a bit on price to your location.
