Well most people I know who've been sexually assaulted/raped haven't reported it to anyone. So it's not about Rape Crisis (for example) under-reporting. It's about women not reporting it at all. (And I've never heard anyone mention being surveyed about it so no idea how widespread that is or whatever.)It's a well known and well-studied fact that the vast majority of rapes and sexual assaults are not reported to the police.
The more trusted figures are collated by a mixture of surveys from various bodies, most of which have no interest in downplaying matters and discussion of methodology is a generally ongoing thing (with the kinds of disagreements over inconsistency of definition etc. that you would expect). Happens at various levels with various kinds of sampling from fairly local to national level, then there are international statistics going right up to UN level. Which gets messy because definitions vary a lot by country but in the UK we have a pretty consistent definition.
I don't think anyone considers the systems perfect and there is a hodge podge of figures but the ones considered reputable don't tend to massively disagree with each other afaik and I'd consider the UK to collect better figures than, say, the UAE.
If Rape Crisis England & Wales were under-reporting by a massive factor people would be justified in taking to the streets.
I think we have no idea of the extent of it tbh. But personal experience tells me that most women have been sexually assaulted in some form at least once.