When removal is genuinely necessary, and when it is being sold to you
Removal roughly doubles the work, so it is worth being clear about when it is actually needed. It is needed when the existing material has been contaminated, by rodents, birds or a roof leak, and when it has compressed so far that it is occupying the space without doing the job. It is also needed when what is up there is unidentifiable and might be something that should not be disturbed casually.
It is not needed simply because the material is old. Glasswool that has stayed dry, uncompressed and undisturbed is still working decades later, and where the only issue is that there is not enough of it, topping up is the right answer and a cheaper one.
If someone recommends a full removal without having established which of those applies, ask what they found. The honest version of this conversation involves photographs from inside the roof space.
What a Victorian roof space actually contains
Rodent activity is the most common unpleasant finding, particularly in older housing and anywhere with heavy tree cover. It is not only the droppings, it is that the material gets tunnelled and matted and stops performing, and no amount of new insulation over the top fixes it.
Water damage is the second, and it usually outlives the leak that caused it. Insulation holds moisture long after a roof has been repaired, and material that is still damp under a new layer becomes a mould problem rather than a thermal one.
The third is bonded asbestos sheeting, which turns up in houses of a certain age around flues, in eaves and occasionally as backing. That is the finding that stops work entirely, and the check for it happens before anything is disturbed rather than after somebody has found it the hard way.
Containment, and why it is most of the job
Decades of settled material carries a great deal of fine dust, and the moment a roof space is disturbed that dust wants to come down through the manhole into whatever room it opens onto. A removal done badly redistributes the contents of the roof space through the house, and the cleaning afterwards costs more than the work did.
Done properly, the access point is sealed and protected before anything moves, the path from there to the door is covered, and the material is vacuum extracted through a line to a sealed unit outside. Nothing travels through the house in bags.
That setup is most of the time on site, which is worth knowing when comparing quotes. A removal priced well below the others is usually one where the setup has been left out.