To be fair, Mr Lammy has not said what you are quoting him as saying.
His tweet appears to refer to 'the will of the people' as being something with which he disagrees, which is his right.
As an MP, a major part of his job is communication, accurate, not open to misinterpretation, and not using unparliamentary language.
Mr Lammy appears to have failed to grasp these essential ingredients, before communicating on TwitFace, and endorsing the opinion of just one voter - albeit a high profile highly paid voter, he is still just one voter with one opinion.
Mr Lammy would do well to avoid President Trump's habit of twitting first, and thinking afterwards - Mr Lammy's position is far less secure!