Owning your own home

What floor(s) is your flat on?

Have you looked through this? https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/443181/BR_PDF_AD_K_2013.pdf

I haven't bothered reading through it but assume it would give some guidance

BCA (building control alliance) guidance. (To the best of my knowledge this is up to date)

Window restrictors
It is often proposed that window restrictors are fitted to window openings
with low cill heights, in order to provide protection from falling.
However, restrictor devices that are commonly fitted to windows would
not be suitable because they can be released (by a key or manually) to
allow the window to open more than 100mm. This leaves the potential for
a window to be left in the open position and people (including children)
would not then be afforded adequate protection from falling required un-
der Part K.
However, “permanent” restrictors (those that cannot be released and
would not allow an opening where a 100mm sphere could pass through)
may be suitable. This type of restrictor, as well as the frame and glazing
used in combination would need to be capable of resisting the loads de-
tailed in BS 6399. However, this may be difficult to demonstrate by struc-
tural calculation, and would most likely require a full scale load test of the
window arrangement fitted with the proposed permanent restrictor device.
The glazing would need also need to provide adequate impact resistance,
typically by the use of toughened or laminated glazing, further guidance
can be found in BS6180.
However, even where the use of permanent restrictors can be justified structurally, they may affect the minimum
purge ventilation requirement under the guidance in the Approved Document to Part F. That guidance requires
a minimum of 1/20th floor area of the room served in openable window (height x width of opening part where the
window opens 30 degrees or more). Where the window opens between 15 and 30 degrees that area needs to
be doubled (i.e. 1/10th floor area of room served).
Therefore, it may not be a feasible alternative unless there is another window in the room that is suitably guard-
ed and can provide the necessary ventilation.
A permanent restrictor would also prevent the window used for escape purposes, so for this to be acceptable
one of the alternative options a) to c) described above under the section titled ‘Conflicts between barrier height
and means of escape’ would need to be provided.

From attached document

TLDR: use of window restrictors can effect minimum ventilation requirements specified in fire regs. They can do one.

Approved Document Part K is the main regulation.
There is a line in it that says:

8.2 Where a person may fall through a window above ground floor level, provide suitable opening limiters, to restrain the window sufficiently to prevent such falls, or guarding.

But it is within a part of the regulation that does not apply to dwellings (which often people don't realise). The fact you have a balcony is not necessarily relevant - perception of risk is different, often windows have a lower bottom edge than balcony handrail, and you might be expected to have a different awareness of eg when your kid is on the balcony, as opposed to just generally in the flat.

People often fit them to domestic windows for reduced risk of falling. However you want to think about whether the window is a potential fire escape window (1st or 2nd floor only).

On the ground floor, limiting the opening is often done on street side so that people don't walk into the open window - again there is a regulation (no protruding more than 100mm or fit a guard rail) but it does not apply to dwellings.

We highly recommend that a qualified window fitter assess the windows and provide advice on any remedial actions"

Sounds like you have documentation that you pre-emptively had a qualified window fitter assess the windows?

windows were fitted and then certified by FENSA. I have a certificate.