Going by that picture, it'll be the big red switch at the left hand end. Fuck knows why that double width black switch with what appears to be an earth leakage reset button has been marked as "MAIN SW" in black marker pen is supposed to do. Feed another consumer unit perhaps?
Each consumer unit only has a single double pole isolating switch to shut off the feed coming from the meter. Once that's shut off, everything is then isolated and safe to work on.
You can normally assume that it's safe to work on if pulling the big switch kills the supply to every electrical circuit in the house.
Obviously, if you have *any* lingering doubts as to how the unit has been wired up, you need to test with a meter or mains voltage probe to verify that the "BIG SWITCH"(tm) has indeed isolated everything from the mains supply and hasn't been bypassed in some 'cunning' way from an alternative link to mains voltage via some other route. Normally, the meter tails going into the terminals of the "BIG SWITCH"(tm) are the only available access to the incoming fused supply cable in a domestic property.
 Whatever measuring device you're using to test for mains voltage, it's important to verify that it will give an indication of mains voltage before you start testing. You can use a multimeter set on a suitable AC volts range, (you have to set it on a range that exceeds the mains supply voltage, typically the 300vac or next higher ac voltage range on the meter) to probe for mains voltage on the PC end of its mains lead with reasonable safety.
You can usually push the meter probe tips into the IEC Live and Neutral socket holes where they'll normally stay wedged in without risk of shorting out before plugging the 13A plugtop end into a switched 13A socket so that you can switch on without actually having to handle the test leads. Once you've established that your measuring device is working as advertised, you can then use it to verify that the CU is indeed free of any nasty surprises before physically handling its wiring.
Incidentally, the best way to probe for (in this case unwanted) mains voltages is to clip the common test lead onto an earth terminal so you only have to hold onto the other test probe's insulated handle whilst carefully poking around inside the CU making sure to keep the rest of your body isolated from earthing (or even other possible sources of live mains) contact. It's best to let the meter do the work rather use yourself as a test probe. :-)
Unless a crazed lunatic installed the CU (or reconfigured it into some unsafe state), pulling the "BIG SWITCH"(tm) (in this case, rather conveniently coloured red), is more than sufficient to render the typical domestic CU and all the fixed wiring it serves (ring main(s), lighting circuit(s), Electric shower, cooker point and immersion circuits, perhaps even another CU) entirely safe to work upon.
If this all seems too complicated and you have any doubts, there's no shame in letting discretion be the better part of valour and employ a professional electrician to do the work for you.
Johnny B Good
Johnny B Good