What will a Hard Brexit do to the british economy, and how will it affect stock prices? why is it so bad?

Many things aren't set in stone yet, like how things are going to affect businesses, whether they'll be taxed etc. So markets may be rather uncertain and will react accordingly with each new policy being passed after Brexit. Currency is one that will undoubtedly be affected by Brexit too, so there'll be a significant amount of movement there too

Time to buy some UK stocks if hard brexit comes :)

It depends alot on whether Britain can negotiate any trade deals with other EU members, especially Free trade agreements. If they cannot do it, then it means that there will be trade restrictions, trade taxes and more, which will impact businesses in Britain. Investors might want to move funds out of Britain to somewhere with better trade regulations.

A hard brexit means trading between the EU block and britain will be affected in the short run because things such aa tariffs amd cross border taxes will be unknown and subject to future rules.

Businesses do not like such uncertainty and they will defer their purchases to a later date when the dust settles