They do boarding checks on most trains these days, so if you wander down early you're likely to get shouted at (unlike with LNR where they don't care, and VT staff don't seem to have any authority to prevent people accessing LNR trains). If you want to be first, find out which platform it is and wander down and wait at the bottom of the ramp just "landside" of the barriers. That doesn't bother them, unlike the control freaks at Paddington who *really* dislike RTT use even if you don't use it to go anywhere you shouldn't before the train calls but simply to find where to stand.
Why not consider reserving a seat? You won't in my view have any difficulty getting one on those trains, but if you want a specific one and you haven't bought a ticket yet buy it on the VT site and use the seat selector?