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Apologies for the late response.
No, your wife cannot stop you severing at any point (she will have no say so whatsoever). i would mark section 7 (c) and mark the second box to state that you have left a copy of the notice of severance.
A notice is simple and can be drafted up online - please use the following tool -https://www.rocketlawyer.com/gb/en/documents/notice-to-sever-joint-tenancy
I hope this helps and please let me know if you have any further questions.