Quote from Animal Welfare Act 2006...
9 Duty of person responsible for animal to ensure welfare
(1) A person commits an offence if he does not take such steps as are reasonable in
all the circumstances to ensure that the needs of an animal for which he is
responsible are met to the extent required by good practice.
(2) For the purposes of this Act, an animal’s needs shall be taken to include—
(a) its need for a suitable environment,
(b) its need for a suitable diet,
(c) its need to be able to exhibit normal behaviour patterns,
(d) any need it has to be housed with, or apart from, other animals, and
(e) its need to be protected from pain, suffering, injury and disease.
(3) The circumstances to which it is relevant to have regard when applying
subsection (1) include, in particular—
(a) any lawful purpose for which the animal is kept, and
(b) any lawful activity undertaken in relation to the animal.
(4) Nothing in this section applies to the destruction of an animal in an
appropriate and humane manner.
10 Improvement notices
(1) If an inspector is of the opinion that a person is failing to comply with section
9(1), he may serve on the person a notice which—
(a) states that he is of that opinion,
(b) specifies the respects in which he considers the person is failing to
comply with that provision,
(c) specifies the steps he considers need to be taken in order to comply with
the provision,
(d) specifies a period for the taking of those steps, and
(e) explains the effect of subsections (2) and (3).
(2) Where a notice under subsection (1) (“an improvement noticeâ€) is served, no
proceedings for an offence under section 9(1) may be instituted before the end
of the period specified for the purposes of subsection (1)(d) (“the compliance
periodâ€) in respect of—
(a) the non-compliance which gave rise to the notice, or
(b) any continuation of that non-compliance.
(3) If the steps specified in an improvement notice are taken at any time before the
end of the compliance period, no proceedings for an offence under section 9(1)
may be instituted in respect of—
(a) the non-compliance which gave rise to the notice, or
(b) any continuation of that non-compliance prior to the taking of the steps
specified in the notice.
(4) An inspector may extend, or further extend, the compliance period specified in
an improvement notice.
Housing a guinea pig with rabbits contradicts all of the points listed in 2, therefore an inspector could advise and make an order for changes in an improvement notice. So they do have some legal rights to make changes to the ways animals are housed.