tbf I agree with the sentiment, but you're only signing on to a manifesto insofar as you're voting for a representative who currently agrees to vote for the things in the manifesto for the most part. They are free to abandon it and leave the party.
and their willingness to do so (or not) should be a big factor in why you vote for someone under the current, deeply flawed, system. It's particularly bad if your MP clearly opposes their party's leader, and you support that leader.
The role of MP is massively overloaded with conflicting and frequently mutually-exclusive expectations in the British system and a vote is a blunt way of expressing preference.