FRS102 affects many things and Section 30 sets out the rules on Currency Conversion.

FRS 102 states that

An entity can conduct foreign activities in two ways. It may have transactions in foreign currencies or it may have foreign operations. In addition, an entity may present its financial statements in a foreign currency

Entities will have a Functional Currency (a concept also used in IFRS) and it allows translation into a Presentation Currency

Reporting at the end of the subsequent reporting periods
30.9 At the end of each reporting period, an entity shall:
(a) translate foreign currency monetary items using the closing rate;
(b) translate non-monetary items that are measured in terms of historical cost in a foreign currency using the exchange rate at the date of the transaction; and
(c) translate non-monetary items that are measured at fair value in a foreign currency using the exchange rates at the date when the fair value was determined.

30.10 An entity shall recognise, in profit or loss in the period in which they arise, exchange differences arising on the settlement of monetary items or on translating monetary items at rates different from those at which they were translated on initial recognition during the period or in previous periods

That all sounds pretty familiar, however, as pointed out by Grant Thornton

Under SSAP 20 Foreign currency translation in current UK GAAP, where matching forward contracts are in place for a transaction, the contracted rate can be used for translation of the matched transaction. This option is not permitted under FRS 102. Instead, a foreign exchange forward contract will be recognised on the balance sheet as a financial instrument at fair value and the associated debtor or creditor will be retranslated at the year-end rate.

A key difference (FRS102) to note in comparison to SSAP 20 Foreign Currency Translation is that SSAP 20 regards consolidated goodwill as an asset of the parent company and not the subsidiary. [Steve Collings Blog]