Earlier quoted context omitted.
MP's are in a hard place because of this. The majority of them are pro-remain, having concluded that brexit is damaging and dangerous and cannot result in a better deal. ... but they will be whipped into line by government (Tory) and party leadership (Labour) insisting that they follow the settled will of the clear majority of the people ... but the will of the people is neither settled nor a clear majority.
How can they be whipped into pary line? Isn't the worst that can happen to them exclusion from said party?
Yes, they could choose to leave the party. Even if they don't feel any duty to said party or to the constituents they're representing (who, realistically, voted on party lines, despite nominally voting for an individual representative), that still means leaving their whole career behind. I would be extremely surprised if we saw more than a handful of MPs do it.