Does changing the plug head of a charger change the wattage?

When using one of the modular chargers in this style:

Can the plug head part be swapped out without changing the wattage of the base unit, or does the plug head itself also determine the wattage the charger can handle?

I ask because I have a 10W iPad charger which unfortunately has the 2-pin plug head shown above (a problem because in the UK plug sockets are 3-pin as in the other picture) and a 5W iPhone charger with the UK plug head I want.

So if I take the plug head off the 5W charger and put it on the 10W charger, will I actually get 10W or will it still be limited to 5W?