Can bailiffs force entry into an outbuilding?

No

Bailiffs do not have authority to break entry to buildings occupied domestically, unless certain circumstances apply.^[1]

If you leave a garden shed unlocked, and the bailiff can reach it, then he can technically take control of goods in the shed.

Likewise, he can take control of a car on your driveway.

If your garage door is unblocked and it has an internal door into the house, the bailiff may enter by lifting the garage door and entering the house.^[2][3]

In practice, bailiffs are not interested in rummaging a garden shed or outbuildings. They just want the money paid, and the threat, usually idle, of turning over an outbuilding might coerce payment.

^{[1] See Can bailiffs gain access with a locksmith
[2] Regulation 20(a) of the Taking Control of Goods Regulations 2013
[3] Paragraph 16 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007}