There are lots of ways to say this politely. I think the best would be "Excuse me, I need to use the bathroom". "Bathroom" is ALWAYS more polite the saying "toilet".

In England we have loads of phrases, such as: I'm just going to powder my nose. I'm just going to spend a penny. (Comes from when public toilets cost a penny to use). I'm just off to visit the conveniences. (A bit old-fashioned this one).

Rose: It is not impolite to say; "Excuse me. I'm going to the toilet." Some might prefer not to use the word "toilet". In that case, you may want to use the term "Ladies Room" . You might prefer the European term, "Water Closet". Or, you can be even more brief, since it is nobody else's business where you are going, and if they ask, you can simply ignore their rude question. You can simply say; "Excuse me." and get up and go.

I should say that 'I'm just going to the toilet' is not in anyway rude and is unlikely to offend anyone. If you want to make it sound a bit more polite you can say "If you don't mind I am just going to pop to the loo" or as George says "the bathroom" (although this is more American English).

I usually say, "I'm going to the Gents" or "I'm off to the Gents". As a female, you could say, "I'm going to the Ladies". There's no "universal" answer for this. Americans tend to favour "bathroom", which sounds odd and coy to my Australian ears. Simply, listen to the people you are talking to and follow what they say. If in doubt, just say "toilet".