IPGN2400 - General principles: tariff quotas
Once a quota is exhausted, the full rate of duty is charged, unless the goods are entitled to relief under other preferential arrangements.
Semi-sensitive goods are restricted by quotas which are fixed by the EU for each calendar year. Most of the goods which are affected by this form of restrictions can be searched for here
For more information on the allocation of tariff rate quotas please see here
https://www.gov.uk/government/collections/uk-tariff-rate-quotas-allocation-of-co-efficients