The 12 kW rating on your oven indicates the maximum power it will draw under any circumstances. The breaker is underrated but it still works because you don't use all functions of your oven at full power for long enough.
Wattage (or power) is the energy consumption over time. One Watt (unit of power) equals to one Joule (unit of energy) per second (unit of time): 1 W = 1 J/s (Consequently, you can also call a Joule a Wattsecond.)
For household electricity, it is common to use Watthours instead of Joules or Wattseconds. Obviously, 1 Watt (unit of power) is one Watthour (alternative unit of energy) per hour (unit of time): 1 W = 1 Wh/h
Of course, you can use the usual SI prefixes, so one kilowatt is 1000 watts and one kilowatthour is 1000 watthours: 1 kW = 1000 W; 1 kWh = 1000 Wh. As there a 3,600 seconds in one hour, 1 Wh = 3,600 Ws = 3.6 kJ.
Wattage (or power) also is the voltage multiplied with the electrical current (or amperage). One Watt (unit of power) equals to one Volt (unit of voltage) times one Ampere (unit of electrical current).
In the UK, the normal household voltage is 240 V. (Nominally, it has recently been changed to 230 V ± 10 %. The actual voltage should still be closer to 240 V for now.) You will generally have a live wire (also known as phase, hot or active wire) at 240 V (alternating current) and a neutral wire (also known as cold wire) at 0 V, which is ultimately connected to ground (also known as earth). This is also true for high-power devices.
In the US, the normal household voltage is 120 V. That's not enough to power high-power devices such as ovens or showers. In the US, you will often have a setup where you have two live wires, one at +120 V and one at -120 V, yielding a total voltage of 240 V.
With alternating current, the voltage is not constantly at 240 V. That value is the average voltage, calculated as the root mean square (rms). The actual voltage is a sine wave with peaks at -339.4 V and +339.4 V (± 240 V * sqrt(2)).
Because it's a wave, the term amplitude comes in. The amplitude is simply the "height" of a wave. There are, of course, different ways to calculate the height/amplitude:
With 240 V power, the rms amplitude (average height) is 240 V, whereas the peak-to-peak amplitude (total height) is 339.4 V - (-339.4 V) = 678.8 V.
The 30 A rating on your oven breaker means that it will allow the connected appliances to continuously draw an electrical power of at *least* 30 A * 240 V = 7200 W = 7.2 kW. It will, however, also allow higher power consumption for a short time. For example, a modern breaker with 'B' characteristics is allowed to provide up to 1.45 times the rated power for up to one hour.
The 12 kW rating on your oven indicates the maximum theoretical power consumption of your oven at any time. Most of the time, it will draw much less power.
While 12 kW is more than 7.2 kW, that does not mean your breaker will constantly trip. You will normally not use full power (all cookers, all functions of the oven) for more than an hour.
Optimally, however, the rating on the oven would be lower than the rating of the breaker (and the wires). Then it would be impossible to trip the breaker in normal operation unless there's a defect.
The 45 A rating on your shower breaker indicates a maximum constant power of 45 A * 240 V = 10,800 W = 10.8 kW. That's enough for a shower rated at 8 kW.