If a company breaches the small company threshold for one year, must it be audited?

No as the company must have breached the threshold for two consecutive years (bullet point 4 below).

S.358 Companies Act 2014 sets out that a company qualifies for audit exemption if it fulfils at least two of the three qualifying conditions listed below as a small company under the Act:

In relation to its first financial year; or
In relation to its current financial year and the preceding financial year; or
In relation to its current financial year and it qualified as a small/medium company in the preceding financial year; or
In relation to the preceding financial year and it qualified as a small/medium company in the preceding financial year