Can I use reflective tape instead of a rear reflector in the UK?
Depends completely on the wording of the regulations in the UK, which are the "Road Vehicles Lighting Regulations" or RVLR.

The relevant points of RVLR are:


Lights and reflectors are required on a pedal cycle only between sunset and sunrise.
Lights and reflectors are not required when the cycle is stationary or being pushed along the roadside.
When they are required, the lights and reflectors listed below must be clean and working properly.
Reflectors have to be fixed to the rear of your bike and to the front and rear of each pedal.
The Pedal Cycles Safety Regulations (PCSR) ensure that every new bicycle is sold with several extra reflectors, some of which are not required by RVLR like spoke reflectors (though these are still a good idea)


Rear Reflector
One rear reflector is required, coloured red, marked BS6102/2 (or equivalent), positioned centrally or offside (on the right-hand side of the bike), between 250mm and 900mm from the ground, at or near the rear, aligned towards and visible from behind.
source https://www.cyclinguk.org/lighting-regulations

Full law is at http://www.legislation.gov.uk/uksi/2009/3220/contents/made

Note that the laws are not retroactive, so if your bike predates 1985 then you are not required to update it.  However this is silly and you should be visible at all times.



For me in NZ, there is a requirement stating:


  You must have a rear reflector that is visible from a distance of
  200 metres when light shines on it (eg a car’s headlights). 
    source: https://www.nzta.govt.nz/assets/resources/factsheets/01/docs/01-cycles.pdf


No part of that indicates what the reflector should be made of, merely that it points backward and has a certain performance.



Yes that tape would fit the legal performance requirements as described.

But it misses out on the labelling requirement, unless your tape says it conforms to BS6102/2 (or equivalent) To be honest anything's better than nothing!

Note you are required to have pedal reflectors too.  Since clipless pedals tend to not have reflectors, check your cleated shoes which should have some of that tape on their heels too.