My understanding is that the £17 fare on National Express is like a full fare ticket on a plane - if you miss one, you just make a call (or go to the res desk ) and get a seat on the next one. I'm assuming the 10% off doesn't affect this.

You talk about lounge access, so I assume you're either flying prem. or have status, so check in at 5am will take minutes and IIRC, security (for which you'll get fast track access) doesn't even open until 5.30am, so seriously don't sweat getting to LHR.