Connecting to a loop-in lighting circuit?

The drawing is a good idea. It probably means that you have far better visualisation skills than most of the other contributors of questions! Unfortunately if you get it wrong, it can raise more questions than it helps to answer.
(1) If the light fittings are double insulated they won't need to be earthed. If they are double insulated they will carry a symbol consisting of 2 "concentric" (for want of a better term) squares - i.e. one square inside the other. It should also state this in the product literature. Metal light fittings and switch patresses (the face plate) should be earthed.
(2) There is no point in retrofitting an earth cable if it is not connected to the main earth terminal at the consumer unit ( and from there to the main earth electrode - earthing rod outside). If you are going to use a 1.5mm2 twin and earth cable, the bare earth should be sleeved with green/yellow sleeving at the switch and at the light fittings.
(3) Your drawing suggests that all of the lights i.e. 1000 W in total will be controlled by the one switch.(At the very least, that switch is going to control 8 lights, not 5!) Is that what you want? You would need to elaborate on your proposed wiring at this "accessible junction box to replace ceiling rose"! A switch needs to have 2 live wires connected to it - the feed to the switch and the switched live feeding the lights. There is one live missing from your drawing and you also show a neutral entering the switch. Except in the case of a double pole switch (which wouldn't apply to this lighting set up) there is no need for a neutral to go near a switch - the switch must always be on the live wire.
But as always, the best thing you can do is get a properly qualified electrician and not mess with it yourself!

J
March 2008