Meter reading hikes up on daily basis even though the main power switch is turned off for days

That sounds like they are using estimated readings. If you send in your actual meter reading it should correct itself. 
Also just a note that this is not a direct route to customer support. If you need to get in touch with OVO then you need to contact them directly. 

Surely if this customer does have a smart meter, then isn't the whole point of having one mean you only get charged for what is used?

That is correct, but regardless of Meter type, you ultimately only pay for what you actually use. However, if Actual Readings are NOT supplied via any means, your bills WILL be estimated and Estimated Bills are wrong 100% of the time because it’s impossible to use Crystal Balls to get it bang on.

Also remember that the original poster is not talking about any billing, merely about what is being shown on the app.

It’s the actual billing at the end of your billing period that matters, not what the app says on any particular day.

What is indicated as daily usage on the app is only that - an indication (a guess in many cases).

Even if you don’t have a smart meter, so there is no way they could possibly know what you have used each day, the app and online account will still indicate (guess) a figure for each day so that they can show a pretty graph.

The app doesn’t know that the OP had gone on holiday and turned everything off, all it knows is that it has no SM reading for those days so it takes a best-guess.
Those guess will be adjusted next time it gets an actual meter reading, either smart or manual.

So unless you do have a smart meter which is sending a (non-zero) reading daily then take what the app or online account says daily with a pinch of salt.