OK, I can see why the UK phraseology only refers to one term - straight ahead. If it this difficult for people to understand the difference between an instruction to fly a heading and an instruction to make good a track I'm frightened to ask a difficult question!
The simple fact is that many controllers use straight ahead and runway heading interchangeably - who knows why, maybe it's laziness, maybe it's because it's only the general direction that matters and although it's scary, maybe they don't recognise the difference. But unless the clearance is ambiguous, do what the clearance says - fly a heading or make good the runway track. If it's ambiguous ask for clarification.
Let's be honest, if there's little wind - or even a moderate amount of wind - it doesn't make any difference anyway whether a pilot adjusts for drift or just flies the runway heading, the controller really isn't going to know the difference!