The activation date you chose seems to be issued as a list periodically by Openreach and isn't updated in real time when that slot is taken by someone else, perhaps on a different ISP.
The "confirmed date" you should get very soon is the one to go by, but as Oldjim said, don't cancel your existing service at this stage.
My broadband basic info/help site - www.robertos.me.uk
| Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 59.4/14.4Mbps @ 600m. - BQM
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.