From my understanding,
In a General Election, the ballots from each polling station are sent to a central 'Counting Room' in each of UK 650 ridings . Once all the ballots in the riding are counted. A riding official will announce the results, along side the candidates (and their huge ribbons). This process could take hours since it could take hours just to get ballot boxes from the more remote locations.
Now, during the referendum ballot boxes will be sent to 382 local count venues.
Unlike in a general election where the numbers of voters is around the same. Local venues will very in size from 700,000 in Birmingham to 1,700 in the Isles of Scilly.
Why does UK count their ballots at these central counting station instead of the polling stations?
My understanding is that they give a lot of authority to the returning officers. I believe there is one constituency where the ballots were counted at the polling stations (in Brighton?)
A few years ago there was a study by the Electoral Commission with respect to election practices, where they studied systems used in other countries, particularly continental Europe. Generally those who did it one way could not conceive that it was possible to do it the other way. The conclusion was counting at central stations worked, so there was no reason to force a change.
Britain has a single registration roll tied to local authorities, so there is no reason not to generally conduct a national election based on those authorities. A parliamentary election could be considered a special case with its own election districts.
It is not too dissimilar to Texas, where elections are conducted by counties, some of which are much smaller than the Isles of Scilly and others are several times larger than Birmingham.
