If you follow the evening votes on any live stream, you'll notice that Bercow has the voting lobbies locked 8 minutes after putting the question.
Here's an excellent outline of how the UK parliament's division vote process works:
In the House of Commons, the Speaker says "The Question is that…", states the question and next says "As many as are of that opinion say Aye". Then, following shouts of "Aye", the Speaker says "Of the contrary, No" and similar shouts of "No" may follow. [...]
If a division is to be taken, the Speaker first states, "Division! Clear the lobbies!". [...]
Two minutes into the division the Speaker puts the question to the House again. If it is clear that a division is still required, the Speaker announces the tellers, two each for the "Ayes" and "Noes". [...]
Eight minutes after the question has been put for the first time, the Speaker declares, "Lock the doors." The lobby entrances are locked, and only those within the lobbies may continue to vote. [...]
After all members have voted in the lobbies, the vote totals are written on a card and the numbers are read out to the House by the tellers. The Speaker then announces these numbers a second time, announcing the final result by saying "The Ayes/Noes have it, the Ayes/Noes have it". The Speaker then adds: "Unlock."