In the SOCPA Act, see below, it says a sound system can be operated solely for the entertainment of the driver or passenger of the vehicle, so presumably if I have my rear mounted speakers facing forward and respectfully ask the other massers not to listen to my 'entertainment' I should be OK on the next ride.
Serious Organised Crime and Police Act 2005 2005 Chapter 15 - continued
137 Loudspeakers in designated area
(1) Subject to subsection (2), a loudspeaker shall not be operated, at any time or for any purpose, in a street in the designated area.
(2) Subsection (1) does not apply to the operation of a loudspeaker-
(a) in case of emergency,
(b) for police, fire and rescue authority or ambulance purposes,
(c) by the Environment Agency, a water undertaker or a sewerage undertaker in the exercise of any of its functions,
(d) by a local authority within its area,
(e) for communicating with persons on a vessel for the purpose of directing the movement of that or any other vessel,
(f) if the loudspeaker forms part of a public telephone system,
(g) if the loudspeaker is in or fixed to a vehicle and subsection (3) applies,
(h) otherwise than on a highway, by persons employed in connection with a transport undertaking used by the public, but only if the loudspeaker is operated solely for making announcements to passengers or prospective passengers or to other persons so employed,
(i) in accordance with a consent granted by a local authority under Schedule 2 to the Noise and Statutory Nuisance Act 1993 (c. 40).
(3) This subsection applies if the loudspeaker referred to in subsection (2)(g)-
(a) is operated solely for the entertainment of or for communicating with the driver or a passenger of the vehicle (or, if the loudspeaker is or forms part of the horn or similar warning instrument of the vehicle, solely for giving warning to other traffic), and
(b) is so operated as not to give reasonable cause for annoyance to persons in the vicinity.
(4) A person who operates or permits the operation of a loudspeaker in contravention of subsection (1) is guilty of an offence and is liable on summary conviction to-
(a) a fine not exceeding level 5 on the standard scale, together with
(b) a further fine not exceeding £50 for each day on which the offence continues after the conviction.
(5) In this section-
"local authority" means a London borough council (and, in subsection (2)(d), the Greater London Authority),
"street" means a street within the meaning of section 48(1) of the New Roads and Street Works Act 1991 (c. 22) which is for the time being open to the public,
"the designated area" means the area specified in an order under section 138,
"vessel" includes a hovercraft within the meaning of the Hovercraft Act 1968 (c. 59).
(6) In Schedule 2 to the Noise and Statutory Nuisance Act 1993 (consent to the operation of loudspeakers in streets or roads), in paragraph 1(1), at the end add "or of section 137(1) of the Serious Organised Crime and Police Act 2005".