This is not really accurate.
Firstly, the majority that initially voted to leave was 37% of the population. As far as I am aware that number has slightly decreased in terms of supporters, but even if the opposite were true it is still less than half of the population that actively support leaving.
Secondly, these MP's are only 'pro-Brexit' insofar as they know it has to happen regardless of their own beliefs and coming out against it now would be political suicide. No matter what you think here, in this last General Election people did not vote for parties based on their Brexit stance. They voted based on whether they wanted 5 more years of Tory rule. Tht issue was a far higher priority than Brexit.
I'd have voted LibDem but you only get one vote and my main concern was "try to halt NHS privatisation" leaving me to vote against the Tories and so for whoever has most chance of winning against them.
There's no scope in General Elections for any nuance in voting.
LibDems openly supported this line, "vote against the Tory party".
Meanwhile in Scotland it seems SNP lost votes not because of their governance, per se, but because people were using it as a proxy for voting against a further Scottish referendum.
This doesn't work so well under the UK's First Past The Post system.
If you wanted to get the Tories out then feeling one was throwing away a vote for the Lib-Dems in a seat were they are too far behind is one argument for proportional representation.
Also the Lib-Dems lost a lot of trust with the coalition with the Tories, many left the Lib Dems as they felt betrayed by this.