They may have the same issue as Zen do on Cityfibre.

The OFCOM speed code of practice requires Talktalk to advertise speeds that are achieved by a subset of test users. This is necessary in different cities across the country.
It needs some with an approved speed tester built in to the router or via a Samknows whitebox.

Until they fully meet these requirements they must advertise the Cityfibre product with the lower upstream speeds available on Openreach.

It might just be that their availability checker doesn't differentiate between the 2 networks so they show the lower upstream to be on the safe side.

There are targeted introductory discounts like this for TalkTalk FF on CityFibre.

https://new.talktalk.co.uk/broadband/city-fibre-1

If that doesn’t load, see the web archive:
https://archive.ph/7oQNy#

550/350 are much better than Openreach FTTP in my view. Well done TalkTalk Business FTTP.

270 down/330 up last night now that TT finally have my connection working. Only took them a week after the fibre went in

I think the AP/Router is still a bit slow, but faster kit will have to wait.