No Deal might damage them in the short term (but, btw, the UK electorate doesn't vote Labour when the economy's in serious trouble - Labour govts are an indulgence set aside for good economic - or at least optimistic - times. Voting Labour during an economic storm would be like a shipwrecked sailor choosing to cling to the anchor rather than the wreckage).
By contrast, extending Article 50 and a second vote would betray their base destroy them forever. Which would you choose?