When studying architecture, mathematics and Physics are the only vital subjects that are required. However, having art or/and history, gives a big advantage to becoming an architect in UK Unis.
So, no, I do not believe the completetion of your Chemistry A2 is required. However, if you chose to study other areas, from medicine to engineering, Chemistry is a very important subject....
So, if you are intent on architecture, then Chemistry is not mandetory, however, make sure that you have a minmum of 3 completed A Levels. Although, I do suggest that having 4 A levels is the healthiest option.