Grats on getting a new place but a couple of things would worry me, being next to the tunnel is asking for trouble as is being a ground floor flat, i don't know the area this is in but i hope all works out well.
Tunnel is for residents of the development only, and this is the first phase, so there won't be any through traffic for at least a year.
As for sound proofing, the only worry that might happen is the flat above us.
Area is a sleepy suburbia, the road is much quieter than most in London, the bus stops running at 8pm. The flat next to ours (other side of our bedroom wall) is the show-flat, and as such will be unoccupied until the entire development is finished (3 years), by when we will probably be looking to move to a bigger place.
I've vetted it as much as I can, and the positives far outweigh the minor negatives. Tunnel means that telly/computer noise aren't going to bother any neighbours, and show flat means our sleep wont be impacted either.
Go in a zone, and a similarly specced flat would be £50k more, back on to a large hospital (noisy!) and front a VERY busy main road.
A similar flat in Zone 2 from the same developer goes for £325k!!!
Much prefer to live in a sleepy suburb, and mission in to town if I want nightlife and stuff. Walk-able distance to a tube station too!