An article I read opined that R Jakobovits was more lenient here than R Mirvis – he would attend when he thought it is beneficial, not necessarily officially requested.
PS If R Jakobovits did not attend that wedding, he was 1) chacham, as over time monarch family would not look favorably at those who favored Diana, 2) thought halachikally that this marriage is not going to last and (not being a navi) presuming a get or an annulment , did not want the melech to have complications doe to presence of kosher witnesses at the wedding