Almost all 13A RCD plugs available in the UK have a trip current rating of 30mA

The 30mA trip rating was decided on many years ago when RCDs first became available and was the trip rating used because at the time many electrical appliances were naturally quite leaky especially appliances which contained Mineral insulated metal sheathed heating elements  The 30mA trip level allowed reasonable immunity from nuisance tripping and a good level of personal protection for the user

It is true that a 30mA RCD will protect you against dangerous shocks and prevent the shock from causing fibrillation of your heart but it is still possible for you to get a really nasty shock without the 30mA RCD tripping

You could for example receive a shock of 20mA which would be very painful and you may not be able to release your grip on the live part but the 30mA rcd would not trip

According to IEC60479, two key levels of electric current need to be considered with regard to shock protection.

The first is the "let-go" level, which is generally accepted to be around 10mA. At or above this level, muscles may seize, and a person touching or holding a live part may not be able to let go of the live part. RCDs rated up to 10mA are intended for protection against "let-go" currents, and are recommended for use in hospitals and old people's homes or similar locations.

The second is the "fibrillation" level, which is generally accepted to be around 50mA. At or above this level, heart fibrillation is likely to occur. RCDs rated up to 30mA are the upper limit for RCDs intended to provide shock protection.

These RCD plugs are 10mA rated so they will protect you against the let go level and the fibrillation level    
If the shock was so mild that 10mA was not reached this RCD would not trip either but you would be below the let go level and could release you grip on the live appliance

Don`t get a nasty shock give yourself a much higher standard of personal protection with 10mA rcd plugs