In the majority of other countries (such as the US, Canada, and many countries in Europe), votes are counted at each individual polling stations. This allows us to get indicative results rather quickly, and usually the counters to go home quite early.
The UK, on the other hand, transports all votes to a counting centre within each constituency, meaning it takes hours to get anything indicative outside the exit poll.
Why do we still take votes to counting centres and not simply count them in the polling stations?