Is there a way to ringfence a cash gift to a married daughter so that in the
event of a divorce at some time in the future, that gift can be excluded
from the joint marital assets, so that it will not then be divided between
the two parties?
Not easily, no. One of the fundamental aspects of marriage is the pooling
of property between the two partners. With the exception of clearly
personal items, such as clothing and low-value items such as jewellery and
electronics, a court will be reluctant to apportion ownership non-equitably
at the time of divorce. That's particularly the case for fungible items
such as money.
More generally, if you want to give your daughter money but with strings
(like these) attached, then you aren't really giving her a gift at all. If
you don't trust her to manage her own financial affairs sufficiently well,
including making correct decisions in the case of some hypothetical future
event, then find some other way of subsidising her lifestyle other than by
means of an outright gift.