This question has been asked before and there doesn't appear to be an advance published list of TalkTalk telephone exchanges sheduled for the upgrade necesssary to support G.fast at the cabinets that Openreach have installed G.fast kit.
Just select the availability checker on the G.Fast page. You may need to clear TalkTalk browser cookies if the page doesn't display.
Just to add that TalkTalk don't offer G.fast straight away for all cabinets although BT Retail and EE (a BT company) are most likely to be ready to offer G.fast ahead of anyone else.
The £28/m price for 160/30 G.fast is nice but the higher speed price starts to sting a bit. How do EE prices compare?
The last locations I had listed for g.Fast readiness from TalkTalk were:
Try calling the G.fast team on 0800 049 2326 (8am-8pm).
Just to say I tried the availability checker for my address and got the expected "not available" message. I then cleared cookies and tried on another post-code and address where g.Fast should be available and I got the same reset result as you did. I conclude the checker is not working as expected.
So do try the telephone number if you're in one of the locations where g.Fast may be available to you.
The EE pricing is better than advertised by TalkTalk for the 330Mbps product (£47 vs £50) but if you can get a TalkTalk offer for the 160Mbps product (£40 vs £28) it's a nice saving from TalkTalk.
Just to add I understand that Openreach are not accepting new orders where the minimum downstream speed cannot support a minimum of 120Mbps.