Thank you for the responses.
DuDuDuLangaLangaBingBong, it is possible to have your beliefs (this isn't just restricted to religious beliefs) recorded when you opt in, as well as stating which tissues you wish to donate, which seems like the most effective way to have your wishes recorded and available if the time comes. I may be wrong but I also believe your family has the ability to overrule opting out (something I personally disagree with), though I couldn't find a government source confirming or disputing this.
SisyphusAndTheRockOfUntidiness, I'm really sorry that you have to put up with a "D"H who behaves that way
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Ereshkigalangcleg, I absolutely agree, I wanted to understand the motivations behind linking to a general opt out when, in my view, it is not directly connected to preventing the use of reproductive tissue. I personally would have my wishes recorded against the collection of my reproductive tissue if this gets close to becoming approved in the UK (unless it is demonstrated as safe for the child, which seems a long way off, if possible at all).