Not quite so simple. First, the principle that the crown is the ultimate authority in all matters (“as well ecclesiastical as civil”) has been fundamental to the English system for nearly 500 years and removing that principle would have implications we cannot possibly scope in advance, given that in a country without a written constitution it will be a factor in statute, in constitutional convention, in legal precedent in a myriad unpredictable ways. Be good for the lawyers, though.
Secondly it is difficult to see how it could be done without disestablishing the CofE, which is something even non-Anglican leaders in Britain, including the Catholic hierarchy, would be very wary of.
Thirdly it might prove a very effective way of stirring a hornets’ nest not only in Northern Ireland but in Scotland too.
Fourthly it cannot practically be done simply by Britain. The British monarch is also monarch of 15 other countries, all of which would need to agree the changes, in many cases pass appropriate legislation identical in effect to British legislation, in some cases (Canada and Australia) in difficult conditions through federal systems, and all timed to come into effect at the same time. Agreement on removing gender discrimination in the succession, and removing the barring of heirs married to Catholics, was agreed by the 16 governments three years ago, and is still some way (probably at least a year) from achieving success.