why would they sell a 2A lamp switch?

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)

Thanks guys,

Some really helpful information here. Flex is blue/brown double insulated and so is the lamp-holder - to fit the lamp-holder to the existing bracket (which is part of the lamp-head tensioning mechanism) I've had to thread its neck (where the wires go in) and use the original threaded brass tube which holds it all together - the wires at this point have extra sheathing around them and the tube ends are chamfered and smooth - even though the risk here is minimal that bit of brass warrants having it earthed. Good to know.

Thanks again.

Wal.

The other thing about fuses is that people talk about them protecting equipment, which isn't usually correct. In fact, the chances are that if the fuse blows, it's because something has already gone wrong! Mainly, the fuse is there to prevent a fire if there is excessive current because of a fault. The nominal fuse rating isn't something to rely on too closely either - a normal fuse will probably carry its rated current for at least tens of seconds before blowing, and probably needs at least twice that to blow quickly. That's fine in practice as it gives just a little bit of headroom for switch-on peak currents but will still blow before the wiring bursts into flame!

For info I just bought a 10A fuse for the plug on an extension cable reel. This was on the high street (Wilko)