Sorry if you were offended by my jocular Welsh-ism.
I've consulted with my Welsh-speaking auntie. She agrees that, although the term 'boyo' is generally believed to be affectionate by the English (like myself), it can be used as a put-down by Welsh people. However it is also used affectionately in a blokey way and is not intrinsically a derogatory word.
If you are Welsh you have reason to be offended. But there was no intention on my part.