Can bailiffs enter by climbing through a window?

No

The bailiff may enter, or re-enter property or premises any door, or any usual means by which entry is gained to the premises.^[1]

Bailiffs must only use a door or usual means of entry to enter premises.^[2]

Bailiffs cannot put his hand in a hole (e.g. a cat flap or letterbox) in order to pull back a bar or fastening to open the door or window because this amounts to burglary and trespass.^[3]

If you leave a window wide open, the bailiff may not climb through it, but if you leave the key in the lock of a door, the bailiff may gain entry by turning the key.^[3]

^{[1] Regulation 20 of the Taking Control of Goods Regulations 2013
[2] Paragraph 59 of the Taking Control of Goods: National Standards 2014, April 2014
[3] Ryan v Shilcock [1851] 7 Exch 72}