HPAC in the midlands are the go to guys, they do most of the air services at events so if you go to one they're at you just hand them your bottle and then collect it at the next event. Some shops offer a hydro service but they don't actually test them, they send them to a test centre and then send it back to you.
The other option is find a diving centre and see if they can get it tested, only places that are IDEST certified can actually test them.
As for price, i'm not sure what the cost is nowadays, last one i had done cost me £20 but that was trade price.