In the UK there is no phraseology for a 'runway heading' departure anymore. This was changed precisely because of the confusion.
Pilots will either be instructed to 'continue present heading' (you may drift off the extended centreline depending upon the wind) or to 'continue straight ahead' (you should TRACK the extended centreline making corrections for drift).
However, the majority of UK traffic will be cleared on SID departures anyhow and the pilot will be expected to follow the track of the route. The clearances above will only be used when required by ATC as part of the big game plan.