Can bailiffs read the debtor's private letters?

No

There is nothing in the Schedule 12 enforcement regulations that enables bailiffs to open mail and read correspondence not addressed to him.

The bailiff is not entitled to rummage anyone's property to satisfy himself the debtor does not live there.^[1]

Bailiffs only have the authority to take control of the debtors goods and sell to pay a debt. The debtors mail and correspondence has no tangible value to bailiffs and cannot be sold, therefore the bailiff is not entitled to interfere with them, and that could attract a liability if the material are to be abused or get into the wrong hands.^[2]

^{[1] See Neil Tunstall vs Direct Collections Bailiffs Limited (DCBL) and Mr Hussain
[2] Section 3 of the Torts (Interference with Goods) Act 1977}