Perhaps think about what it is you like about each subject; for example, if you like the subject matter of classics, and the methods, although not necessarily periods, studied in a history course, then an ancient history degree might allow you to study the periods you are interested in using historical methodologies. If however you are interested in the periods of modern history you are studying, but enjoy e.g. the literary or language aspects of classics, you may want to consider joint honours courses in history and languages (perhaps taking up a modern language), or history and English.
There are however courses combining ancient history and (modern) history though, up to and including Oxford (Ancient & Modern History) and Cambridge (the history course at Cambridge allows you to study ancient history as well as modern history).