Do bailiffs close gates and doors once inside so pets cannot escape onto the streets?

No

There is no enforcement regulation that requires bailiffs to contain pets in the property.

Common sense suggests bailiffs ought to be sure that animals cannot escape, otherwise they could be at risk of a claim for damages in the event of an accident.

Bailiffs cannot take control of livestock,^[1] including livestock contained in a vehicle on a highway,^[2] and in any case, bailiffs cannot take control of domestic pets.^[3]

If bailiffs allow animals to escape causing damages, a claim may be brought.^[4]

^{[1] Regulation 11(2) of the Taking Control of Goods Regulations 2013
[2] Regulation 11(4) of the Taking Control of Goods Regulations 2013
[3] Regulation 4(1)(c) of the Taking Control of Goods Regulations 2013
[4] Section 3 of the Torts (Interference with Goods) Act 1977}