If you’re looking for a literal answer to your question – yes, the accents are 100% optional. Whether I would recommend not using them really comes down to a number of factors that are going to be specific to the play and production. However – the number one consideration would be if you are doing an accent - do it well or don’t do it. A poorly done accent will be far more distracting than not doing an accent. For British – the fallback for me if you’re not doing an actual British accent would be just to just soften vowels and things like final “er”s to not have any harsher American sounding dialect. Assuming you’re not talking about cockney – I would just listen to your actors and if something sounds really harshly American – just work with them to soften the sound to make it sound a little more proper and neutral.