Firstly, May with vocal support from these same Brexiters asserted that she would just go ahead and issue Article 50 when it suited her. A private citizen then went to court saying that it required a Parliamentary vote, Brexiters again whined. The lady one and the Supreme Court affirmed the lower court's decision, more whining.

Secondly, the House of Lords is a fundamental part of the British Parliamentary system. You can call it what you like but it is simply exercising its rights.