Unfortunately in the current environment of perusing legal action for absolutely anything & everything.
I'd sooner be legal in the event of an incident. And knowing the law might help you defend yourself if you can point out to any police at the scene that their vehicle is not road legal.
Road Vehicles Lighting Regulations 1989
http://www.legislation.gov.uk/uksi/1989/1796/contents/made
This legislation includes pedal cycles. But its from 1989 so may have been revised or superseded by EU law so best to check before you follow this!
First exemption:
(3) Nothing in these Regulations shall require any lamp or reflector to be fitted between sunrise and sunset to–
(a)a vehicle not fitted with any front or rear position lamp,
(b)an incomplete vehicle proceeding to a works for completion,
(c)a pedal cycle,
(d)a pedestrian-controlled vehicle,
(e)a horse-drawn vehicle,
(f)a vehicle drawn or propelled by hand, or
(g)a combat vehicle.
Daylight hours is legally defined as the time between half an hour after sunrise and half an hour before sunset.
(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.
23. (1) No person shall use, or cause or permit to be used, on a road a vehicle unless every lamp, reflector, rear marking and device to which this paragraph applies is in good working order and, in the case of a lamp, clean.
11 Any other lamp - Used so as to cause undue dazzle or discomfort to other persons using the road.
Looking at the schedules at the bottom of the page any bicycle manufactured after 01 Oct 1985 must have amber pedal reflectors. Before this they are optional.
This is the only thing my bicycle doesn't meet the legislation on at the moment as my SPD pedals don't have them.