why would they sell a 2A lamp switch?

[pinballdave]

The 2A rating on the switch is the breaking capacity of the switch. That is that it will reliably switch off a current of 2A without the inevitable arcing across the switch contacts welding the contacts closed. The switch itself will be safe to carry higher currents without damage if it remains in the on position, far higher than is necessary to blow a 3A plug top fuse.

As m_c said the only over-current situation you are protecting against by choosing a lower value fuse, is an overload between 2A and 3A occurring while the switch is being turned off. As the lamp holder is designed for lower wattage bulbs (max 100-150W) and I don't believe there are bayonet fitting 500-750W bulbs easily available that could be used, you can state that the design load is inherently low enough to ignore this.

If the lamp holder is fully insulated (has the box inside a box 'double insulated' symbol on it, then you don't need an earth cable. The flex must also be double insulated (ie brown/black insulated wires inside an insulated sheath). If the flex is only single insulated it _must_ be replaced, also if it's black/red then it would be wise to replace it with something a bit newer.

Dave.
(16th edition qualified electrician)