The rules ARE national, and are very simple.
It has nothing to do with terminology about merging etc.
It is and has always been about lines of and lanes of traffic.
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148 Giving way when moving from one marked lane or line of traffic to another marked lane or line of traffic
(1) A driver who is moving from one marked lane (whether or not the lane is ending) to another marked lane must give way to any vehicle travelling in the same direction as the driver in the marked lane to which the driver is moving.

 149 Giving way when lines of traffic merge into a single line of traffic

 A driver in a line of traffic that is merging with one or more lines of traffic travelling in the same direction as the driver must give way to a vehicle in another line of traffic if any part of the vehicle is ahead of the drivers vehicle.