Yes, I think you are overreacting.
Your therapist is not her husband. The relevant question is whether she is professional, boundaried, competent, ethical, and helping you. If the therapy has been working for a year and you liked her yesterday, it seems a bit extreme to suddenly decide she is unsuitable because of political views you are assuming she shares by marriage.
Also, the EV and solar panels stuff feels irrelevant. This is therapy, not a hustings. People can disagree about net zero, immigration, Brexit, the EU, or Farage without being personally unsafe, immoral, or incapable of doing their job.
I also think there is a lot of assumption here. Reform’s position is not simply “we hate immigrants”. Whether people agree with them or not, there is an obvious distinction between legal migration, illegal migration, settlement rules, asylum, deportations, EU membership, and net zero policy. Lumping all of that together as “she married a Reform man, therefore I can’t sit in a room with her” is not exactly nuanced.
What happened to tolerating people who do not share every political view? What happened to grey areas? If she has said or done something in therapy that makes you feel judged or unsafe, that is different. But if the therapy itself has been good, I would judge her on the therapy she provides, not on a political identity you have projected onto her via her husband.