It all depends if you're watching content from On Demand or from a recording.
A recording from a broadcast channel should always show where you are in a show during fast forward or rewind.
However depending what On Demand content you're watching, it doesn't always do that, for instance in Sky Cinema On Demand won't at the moment. This is because different On Demand sources use different technology, some of which can show a preview and some currently can't.
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I work for Virgin Media - but all opinions posted here are my own