If you want to avoid lawyers and limit lawsuits, the best course of action is to base your opera on public records, existing news articles, and other public facts.
Facts are free. The facts of Dorothy's life are free for all to use. This is true under U.S. and UK law. So, for example, you could dramatize the pertinent facts of Dorothy Miles's life - for example, how she was born with hearing but lost it after contracting meningitis, or how she fell in love with American sign language, and later combined British sign language and poetry, or how she suffered from depression and bipolarity and that led to her death. This use of public facts often provides the springboard for life-story dramatizations, as in the Dorothy Miles documentary, Dot.
Using Dorothy Miles's poetry and writings. If you want to use material taken from Dorothy Miles's books or poetry you will need to investigate its copyright status. Under U.S. and UK law, permission is required from whoever acquired rights from Miles after her death, perhaps Miles' niece, Liz Deverill, or perhaps the British Deaf History Society, the publisher of Miles' works. We couldn't find any copyright claims for Dorothy Miles in the Library of Congress (USA) so you may need to investigate copyright status at the UK's Intellectual Property Office. The rights that you seek for Miles's writing can be narrowly or broadly defined, for example, rights limited to the live opera performance, or rights to also distribute and stream the opera. Similarly, you would need permission to use video clips of Miles, for example, if you plan to display those clips in conjunction with your opera.
Living people. If you plan to identify and portray living people you don't need their permission under U.S. law unless you are trying to prevent three types of legal claims: right of publicity, defamation, or invasion of privacy. We don't imagine this will be an issue for you unless you are casting a living individual in a negative light or unearthing secret information about a private citizen. (We're not knowledgeable about British law but it's our understanding that the UK has no right of publicity.)
Adopting source material. If you're adopting source material, for example working from a biography and taking more than facts, you'll need a derivative rights agreement from the copyright owner. If you are using source material obtained directly from an individual portrayed in your opera, you may need either a release, a consulting agreement, or in some cases life-story rights. The latter agreement prevents lawsuits over fictionalization and releases the producers from any claimed injuries resulting from how the character is depicted. Hopefully, if you follow these suggestions, we'll soon see your opera on this list.