"How can I be sure property will increase in value?"

You can't. That's the risk factor. The downside of all investments is that there is some element of risk.

However, property is regarded as "low to medium risk" because historically it has always increased in value over a reasonable time frame.

Of course there are logical reasons for that. People need shelter. Land is becoming increasingly hard to come by. The population is increasing. And of course there's the effect of inflation.

Historical evidence, combined with a common sense analysis of the present situation is the best way to predict the future.