Hi Carl, welcome to the forum :)
You'll find the members on here are a friendly and helpful bunch :)
You'd be exactly right in saying that the only difference with the s line is lowered suspension 30mm I believe) and the body kit, plus a slightly different interior. The wheels should bolt straight on and plenty of TT owners have done this.
If you can't order the car with the wheels, sometimes the dealer will allow you to upgrade once the car has arrived. Almost like having accessories fitted at a later date such as mudflaps etc. If that makes sense.
Your best bet would be to speak to the dealer you ordered the car from directly with this matter. Or potentially their service department.
Sorry I can't help more but hope IV helped a little :)
Cheers
Steve