Hi. Obviously with airport security etc. it's impossible to give precise answers, but what are the odds I could get through customs (UK citizen arriving from Spain) at the South terminal and to the train station in around half an hour at 8pm on a Saturday?

I'm thinking it's not great odds, but I don't really want to wait for 90 mins for the next train if I can get that earlier one! (I know they are far more frequent than hourly, but the onward connection in London isn't).