It may be that you've already done this transfer and discovered the answer yourself (!), but it is a UK Government requirement that all departing passengers are security screened to UK standards. This includes arrivals from the EU and the UK's Common Travel Area partner, the Republic of Ireland. The only transit passengers who do not have to pass through secondary security are those who have originated from a UK regional airport (plus Belfast in Northern Ireland, which falls under the UK standards), so OSS will not be implemented in the UK - probably ever.