If you have Cash Accounting turned on, as a lot of SME's do, Sage does not allow you to use the Foreign Trader module, which is Sage's way of managing multi currency.
However, if you are using OpenCRM and have set multi currency up correctly in the OpenCRM settings, you do have an option that might help.
In OpenCRM you can set the Company to have a default currency, this helps because it sets financial entities (Quotes, Sales Orders, Invoices and Purchase Orders) with the correct default currency and VAT/TAX rate.
When the financial record is saved in OpenCRM a number of fields are updated, this includes the Base Currency fields for your installation. So even though you did not enter the values for this company in GBP, the GBP converted values are saved. This is particularly relevant and true of Invoices and Sales Orders.
When SageLink imports this Invoice into Sage Accounts 50 it can be set to import the GBP values as this is all that Sage will understand (without the foreign trader module turned on). This amendment to the export/import in SageLink is necessary as Sage treats the figures as numbers, as opposed to a specific currency and then applies these figures with the assumption that they are the Base Currency, which may not be the case.
When you receive payment into your bank, presumably as a GBP payment, this can be entered onto Sage as a Sales Receipt, and any adjustment required should be made (if the exchange rate is different to the OpenCRM exchange rate set at the time of the Invoice being created).