Technicaly LEDs are illegal as the law mentions something about a Filament lamp, which applies to all vehicles. So some modern cars break the law. As for flashing that is legal now
I’m going to say that’s balls.
So back it up with something more intellectual then !
Or just in case you cant extracts from the regs
Pedal cycle
Type of lamp Schedule Exceptions
Rear position lamp Schedule 10: Part I None.
Filament lamps14.—(1) Where a motor vehicle first used on or after 1st April 1986 or any trailer manufactured on or after 1st October 1985 is equipped with any lamp of a type that is required by any Schedule to these Regulations to be marked with an approval mark, no filament lamp other than a filament lamp referred to in the Designation of Approval Marks Regulations in–
(a)regulation 4 and Schedule 2, items 2 or 2A, 8, 20, 37 or 37A; or
(b)regulation 5 and Schedule 4, item 18,
shall be fitted to any such lamp.
(2) Where any pedal cycle manufactured on or after 1st October 1990 is equipped with any lamp that is required by any Schedule to these Regulations to be marked with a British Standard mark, no filament lamp other than a filament lamp marked with the marking indicated in the British Standard specification for Filament Lamps for Cycles published by the British Standards Institution under the reference 6873: 1988 namely “B.S. 6873” shall be fitted to any such lamp.