Chris, I forget what camera you have (was it an 1100D?)...
The Jessops flash has a slave mode so you can use it off the camera but you'll have to use another flash to fire it. You can use the pop up flash on your camera to do that but you will end up effectively using 2 flashes that way (the gun and the pop up flash).
You'll also have to control the flash manually if you use it off camera. You're camera won't be able to meter for it won't know it's there and won't compensate it. This may not be a problem but you should be aware of it.
Some of the newer Canon cameras have a built in flash commander which you can use to control everything straight from the camera but I don't know which camera you have so I don't know if you have access to that system.
I've never used the Canon off camera flash system but if it's anything like the Nikon CLS it's very powerful and lets you control multiple flashes in several groups, which you can adjust individually, all from the camera. Of course, you'd have to have compatible Canon flashguns.
For me, the best basic off camera flash system you can get is the
Nissin Di466 (which is completely Canon compatible) and costs the same as the Jessops flash. The main reason I suggest this flash is that I've heard nothing but complaints about Jessops flash units and I've used Nissin strobes on both Canon and Nikon systems and they've worked flawlessly.
You'd need a trigger to fire it off the camera which has the significant advantage of not having the pop up flash showing in the photo. These don't have to be expensive. You can do it with a cable for a few quid (depending on specific equipment) but if you want to go wireless, I'd suggest the
Interfit Strobies i Sync. It's £45 and will work using both IR and radio which means you can fire almost any flash off camera.
It's still manual, which means you have to adjust the camera and the flash individually rather than relying on the camera to know the flash is there and ajust everything to the correct exposure but it's the most flexible and powerful system you'll get without spending hundreds on a proprietary system.