Reflective sheet
I've done a quick bit of 'digging' on this subject and it appears that any reflective material on the back must reflect red light only. Here's the legislation bit if it helps!
Regulation 11(2) of the Road Vehicles Lighting Regulations 1989 provides that no vehicle shall be fitted with a lamp or retro reflective material which is capable of showing any light to the rear, other than a red light with certain exceptions.
11(2) No vehicle shall be fitted with a lamp or retro reflective material which is capable of showing any light to the rear, other than a red light, except
(a) amber light from a direction indicator or side marker lamp;
(b) white light from a reversing lamp;
(c) white light from a work lamp;
(d) light to illuminate the interior of a vehicle;
(e) light from an illuminated rear registration plate;
(f) light for the purposes of illuminating a taxi meter;
(g) in the case of a bus, light for the purposes of illuminating a route indicator;
(h) blue light and white light from a chequered domed lamp fitted to a police control vehicle and intended for use at the scene of an emergency;
(i) white light from a red and white chequered domed lamp, or a red and white segmented mast-mounted warning beacon, fitted to a fire service control vehicle and intended for use at the scene of an emergency;
(j) green light and white light from a chequered domed lamp fitted to an ambulance control vehicle and intended for use at the scene of an emergency;
(k) blue light from a warning beacon or rear special warning lamp fitted to an emergency vehicle, or from any device fitted to a vehicle used for police purposes;
(l) amber light from a warning beacon fitted to:
(i) a road clearance vehicle;
(ii) a vehicle constructed or adapted for the purpose of collecting refuse;
(iii) a breakdown vehicle;
(iv) a vehicle having a maximum speed not exceeding 25 mph or any trailer drawn by such a vehicle;
(v) a vehicle having an overall width (including any load) exceeding 2.9 metres;
(vi) a vehicle used for the purpose of testing, maintaining, improving, cleansing or watering roads or for any purpose incidental to any such use;
(vii) a vehicle used for the purpose of inspecting, cleansing, maintaining, adjusting, renewing or installing any apparatus which is in, on, under or over a road, or for any purpose incidental to any such use;
(viii) a vehicle used for or in connection with any purpose for which it is authorised to be used on roads by an order under section 44 of the Act.
(ix) a vehicle used for escort purposes when travelling at a speed not exceeding 25 mph;
(x) a vehicle used by the Commissioners of Customs and Excise for the purpose of testing fuels;
(xi) a vehicle used for the purpose of surveying;
(xii) a vehicle used for the removal or immobilisation of vehicles in exercise of a statutory power or duty;
(m) green light from a warning beacon fitted to a vehicle used by a medical practitioner registered by the General Medical Council (whether with full, provisional or limited registration);