I'm not sure I understand, British capitalists were the same before, during, and after Thatcher. They weren't allowed to be competitive prior to Thatcher because so many industries were nationalised, for example auto manufacturing and aerospace.

It was Thatcher who divested and deregulated all those industries, removing central government from being involved in any businesses such as trains, home building, aerospace, etc. Rather than be forced into deals with labour unions, British capitalists were now free to deal in the global market, and immediately began closing British factories because they were uncompetitive and the government was no longer forcing them to remain open or paying subsidies.