What? So we should go ahead with Brexit in order to appease the far right? As reasons for Brexit go, that has to be one of the silliest.
And I'm not a great fan of McDonnell, but to describe him as a far left extremist is pretty absurd.
It's not so much appeasement of the far right as cutting them off, sometimes a moderate competitor is the best solution, as in the case of a Conservative-led hard Brexit. After all, the Conservative Party will be in a position to take the credit if Brexit goes through as planned, especially a no-formal-deal ( the risk of damage to the EU means that they will do something at their end to keep everything moving).