Pengggwn (apologies for mis-spelling the first time), you do not need to buy the book, that was not my point. My advice is coming from being previously involved in a related situation. Obviously, I can't give any details other than to say that the law is very clear: In the event where the mother cannot give consent for whatever reason, her next of kin do not have legal authority to decide for her unless this has been pre-arranged in an enduring power of attorney or similar legal document.
The birth plan communicates the express written wishes of the mother regarding her care/treatment during labour. The fetus has no rights in the UK, Canada and many other jurisdictions until birth (I'm assuming the OP is in one such country). In a situation where the mother is unable to give consent, the mother's legal right to not receive treatment that she has clearly documented that she does not wish to receive, are paramount, over and above the wishes of her next of kin and the rights of the fetus, i.e even if it means risking severe damage to fetus.
So in the example where the mother documents that "under no circumstances do I consent to forceps" or something equally strongly worded and then gets "out of it" during labour for whatever reason, if it comes to a point where forceps are the best option but she is in no position to decide, the legally correct thing to do is to take her to theatre for a C-section, no matter how late in the piece. I am not trying to pick an argument. Categorical statements in birth plans have caused problems. Granted, this is NOT common, but it doesn't mean these problems don't happen ...sadly, they do and with devastating consequences for all involved.
OP - a birth plan is important, you need to know what the possibilities are and what the consequences can be, you may feel strongly about some interventions and you have every right to express that, just don't back yourself into a corner - use the template and ask your maternity care professional for advice with wording if unsure.