Final answer:
The United Kingdom is taking part in a Pegged exchange rate system when it enters into an agreement with other countries to keep the exchange rate among their currencies fixed.
Step-by-step explanation:
When the United Kingdom enters into an agreement with other countries to keep the exchange rate among their currencies fixed, it is taking part in a Pegged exchange rate system. In a pegged exchange rate system, the government intervenes to manage the value of the exchange rate and keep it fixed to a specific level. This is different from a floating exchange rate system, where the foreign exchange market determines the rates.