That's an incredibly cheap jewellers! Most charge at least £8, with many wanting considerably over a tenner. (However there's a local market stall around here which replaces batteries for £3).
If the rim of the back of the watch has two (or more) notches in it, that means that it's a screw fit. It's possible to unscrew the back by pushing hard on the edge of one of the notches (anti-clockwise) with a small screwdriver. However it's also easy to end up stabbing yourself with the screwdriver!
If there are no notches, it's simply a push fit. The blade of a penknife can be used to prise the back off, with a quick twist. (Once again, it's also possible to injure yourself. Take care!).
Chris