if any trains will run on that day the individual train companies (in this case Greater Anglia (who ran no trains at all to Cambridge in last month's strike), Great Northern and Thameslink) will post what is called an Emergency Timetable (sometimes called a Strike Timetable) listing all the trains they hope to run after consultation with the unions and company management, two or three days prior to the strike. They won't necessarily meet all aspects of that timetable.
Coach (will be very full probably), and not easy, or car (either an airport service by a Cambridge minicab company or a car you rent and drive yourself (not always a good idea on busy motorways (Heathrow to M4 to M25 to M11 to Cambridge or Gatwick to M23 to M25 to Dartford Crossing (£2.50 paid online) to M25 to M11) )right after a long flight which is tiring and likely under the effect of jetlag) are likely the other choices.