For a double insulated lamp with bayonet lamps it does not really matter which connects to line and which connects to neutral. With a screw lamp holder it does however the centre contact should be line.
In the UK it is normal to use the ceiling rose as a junction box this has quite a few advantages to using the switch as a junction box. 1) Less volt drop. 2) Power for emergency lights or ceiling fans. For example so the ceiling rose will often have three cables plus the flex to lamp. There will be:- 1) Supply in. 2) Supply to next lamp. 3) Switch wires. Sometimes there may be more but the ceiling rose will as standard have four connections. 1) The earth is normally a clamp. 2) Pair of connectors which has wire from switch and wire to lamp cord. 3) Centre three connections which has all three cables connected to but does not connect to lamp, these are always live. Are used for fans or emergency lights only. 4) Neutral normally three connectors which includes connection to bulb.
When fitting a lamp instead of the ceiling rose either the ceiling rose needs replacing with a junction box or the lamp needs the four connections even if it only used two. The normal is to replace the connector block with a four way connector block but clearly getting three wires in the block is not easy.
The main problem for the DIY is the wire to switch is often the same as others but both cores are line so you have a wire the same colour as neutral but it's not a neutral. In theory it should have a sleeve on it but often they fall off. So be very careful not to get it mixed up.
As with most Q's and A's on here a diagram or two is a must. I use a site called 'Flameport'. There are lots of diagrams, which can be enlarged if needed. Wiring is shown in older and harmonised colouring. Have a look.
Sorry this reply is late, but I thought suggesting this site would prove useful in future.
I have a library of diagrams which on other sites I link to but with this site it will not work with many of the formats and does not seem to like links.
With other sites after I post I can view the post and make any corrections but with this forum one can't even see your own posts.
It does however stop the rude posts and it also means any errors are the responsibility of the system operator to spot rather than the poster so one hopes there are less errors.
With the other forum I use I have a selection of images stored on the forum I just click show images and select the one I want.
But I feel to use images which are then don't show is a problem and I do not want to click on pre-view with every post so until the poster can see and amend their own post directly after posting sorry I am not messing around with pictures. If you want pictures don't use this forum.
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