BIOS on HP and other large OEM are normally locked down, so there will be no options for overclocking.
Even if there was, then the I5 2400 has a locked multiplier, so overclocking would be limited.
The only available option if you need to boost performance of this PC (rather than just buying something newer), is to buy a second hand I7 2600 (fastest processor officially supported on the Elite 8200) which can be had for around £20 from CEX or Ebay.
(The i7 2600 has an extra 300mhz base clock, and 400mhz boost clock over the i5 2400, as well as an extra 2Mb cache and Hyper-threading that the i5 lacks)
(The i7 2700k would be slightly faster and should also be compatible but it wasn't officially offered, and is normally a lot more expensive, around £40-£50)