The UK version of QuickBooks Premier has a report called "UK Standard Balance Sheet" under "Reports > Company & Financial".
However, if I send a QBXML GeneralSummaryReportQueryRq request for a BalanceSheetStandard I get a different report simply called "Balance Sheet", which is ordered differently and has a couple of extra rows compared to the "UK Standard Balance Sheet".
This "Balance Sheet" report does not seem to be available from the "Reports" menu in QuickBooks, but it does get displayed if I set <DisplayReport>true</DisplayReport> in my QBXML request.
Is it possible to request the UK-specific report via QBXML or am I stuck with this other report that it's currently returning?
You are stuck with what is returned. Reports in the SDK are limited. You might try BalanceSheetSummary and see what it returns.