Graham - I have found it hard to get specifics. there is a standard for reflectivity and for size I think - tape should be able to meet it.
Just been looking. The relevant law seems to be Section 18 of the The Road Vehicles Lighting Regulations 1989 which basically says you must use the light and reflectors specified in the table.
For pedal cycles that table says we need a "Front position lamp, Rear position lamp, Rear retro reflector and a Pedal retro reflector".
The definition of a "Pedal Retro reflector" is in Schedule 20 and says:
Number: Two reflectors on each pedal
Longitudinal: On the leading edge and the trailing edge of each pedal
Angles of visibility: Such that the reflector on the leading edge of each pedal is plainly visible to the front and the reflector on the trailing edge of each pedal is plainly visible to the rear
Markings: A British Standard mark (specification for photometric and physical requirements of reflective devices published by the British Standards Institution under the reference BS 6102: Part 2: 1982, namely “BS 6102/2”)
Size of reflecting area: No requirement
So there you go. Technically tape probably wouldn't meet the requirement as it won't have a British Standard Mark.
Though if you had tape as you described you would hope any sensible judge would tell them to stick such technical objections where the sun doesn't reflect.