60950-1 allows a given potential, and its associated clearance, to be broken up into N+1 gaps of width (1/N or minimum gap, whichever is greater). That is, the total gap always meets the minimum, even in a single failure event.
This is most applicable to chains of resistors, but the same logic applies for LEDs, yes.
It would be rather stupid to not be able to get 7 mil clearance between traces that are 10V apart, just because they're both floating at 300V. This standard, at least, allows for such.
Writing accurate design rules, to reflect these considerations, can be very difficult. It is probably better within the OP's capabilities to simply make a board as he pleases, submit it to the local approval group (the UK's equivalent of UL) for review, and revise until correct. (Again, this is a very expensive procedure, but it seems to be no more expensive than other processes the OP has revealed, and is more likely to be correct than free information over the internet here.)
Tim