I think most people in the UK who watch cookery programmes will probably have heard the word "patissier" and would know what it means. I would say it's part of the English language, yes, but not necessarily that everyone would know what it meant.
We pronounce it approximately the same way as it's pronounced in French. I don't do phonetics but I would describe it as "pa-teece-ee-ay".
Edit: As BobK pointed out, we perhaps don't pronounce it exactly the same as the French do but I wanted to make it clear that (in the UK) we don't completely Anglicise the pronunciation. If we did, we would end up with something that sounded like "put-iss-ee-yer" or "pat-iss-eer" or even "pat-iss-eye-er". If a British person with absolutely no knowledge of French pronunciation saw the word for the very first time, I'm not sure how they might attempt to say it.
As Bob said, the final "-ier" is more like one single syllable in French, giving something closer to "pat-eess-yay".