yes you are right that B1 test can replace the need of A1 test as an alternative because it is higher in level and even meet the ILR & British citizenship requirement. But one more dilemma still unresolved about it that whether will it be unacceptable after its 2 years of expiry or will remain acceptable in next extension application. As per following home office guidance under Under paragraph 32D of Appendix FM-SE previous english test remains valid. Whether spouse visa entry clearance/extension falls under Under paragraph 32D of Appendix FM-SE.
https://assets.publishing.service.gov.u ... n_1.21.pdf