Just to clarify:
Cash invested in AVC's won't be reduced in the vast majority of cases, if it's taken before the age you selected.
The only possible scenario I can think of when it might be reduced, is if you have AVC's invested in 'With Profits' products via the legacy AVC providers of Equitable Life & Standard Life(there maybe more, but can't remember off hand).
With Profits products often have a set access age, which you probably would have chosen when you first took out the product.
You can still either access or transfer your cash earlier, but it's usually subject to a Market Value Reduction, which will probably vary with the provider and your age, etc.
Zurich took over the admin of AVC's in 2010 if I remember correctly, followed by Scottish Widows in 2019, but some people may still have cash tucked away with those legacy providers.
Income Tax is also a potential way your pot may be reduced, but that is dependant on specific amounts involved, so might catch some out.
Personally I expect to pay tax on part of my AVC's because I have managed to build more than 25% of pot value. The excess will be paid out as a UFPLS, meaning the first 25% of the excess is also tax free.
As long as I stay within the 20% tax band in the year I take that excess, I'll still keep 85% of it.
As far as I'm concerned, 85% of something is better than 100% of nothing.