Hi OP, I was on topiramate, prescribed for me by the GP for a long time before I ever got to see a neurologist. TBH it did fuck all except give me appalling side effects, but I am aware it works for other people.
My DH is also infertile (bugger all sperm in the samples, absolutely nothing capable of swimming - what was in the sample was either dead or so mutated it couldn't actually move in any direction) due to being a premature baby and subsequent issues with undescended testes so everything was literally cooked.
If I recall correctly, I also lied and said we'd use condoms and a diaphragm with spermicide :-) Then used nothing as usual.
I do actually have the Mirena coil now to deal with perimenopausal symptoms, and to try to see if it helps with my chronic migraine (nope, but I'll try anything!).
So my advice is lie to the GP. The BNF guidance on women of childbearing age and topiramate states "Manufacturer advises perform pregnancy test before the initiation of treatment—a highly effective contraceptive method is advised in women of child-bearing potential; patients should be fully informed of the risks related to the use of topiramate during pregnancy."
https://bnf.nice.org.uk/drugs/topiramate/
I'd go back to the surgery with that link, the quote and your description of your (imaginary) contraceptive plans.
Whatever happens, please please check the list of side effects and be aware of them - I was an absolute zombie after 2yrs, it nearly cost me my job and my marriage because I was such a mess. The GP was worried I had blood cancer or something but we finally worked it out..... I'm not allowed it again!
For hemiplegic attacks, it was actually Botox (via neurology) that got on top o those for me. I'm down to a couple a year from two a week....