There are a number of preliminary breath screening devices routinely used by the police that are approved not only under the Road Traffic Act but also under the Railway and Transport Safety Act 2003
These include the Draeger Alcotest 6810 GB, The Lion Alcometer 510 B and the Envitec AlcoQuant 6020.
These devices are not approved for evidential use but would provide sufficient information for a police officer to arrest and then require a sample on an evidential device at a police station.