I have been caught speeding (53mph in 30mph zone) and driving with an
invalid licence(south african licence only valid for one year in UK).
I am due to appear in court. I am on the verge of bankruptcy and
cannot afford to pay a heavy fine. 1) What is the worst case scenario
regarding the punishment for this? 2) Is there any way to reduce any
fines(there will obviously be a fine)? Where can I go for online
advice that is not going to be too expensive? Thanks.

You need to see a solicitor. This is a case where you need specialist
advice.  For some general advice, your local Citizens' Advice Bureau
will be able to give free help.
But you are likely to face both a fine and a driving ban.