IIRC TT specify the middle or most stable of Openreach's line profiles. BT Wholesale default to the Fast(est).
BT Wholesale do not default to any DLM profile. Maybe you meant BT?
BT Wholesale use what the ISP asks them to use.
BT Wholesale use the same 3 DLM profiles OpenReach use (they have to, it's OpenReach's DLM) but the have their own 3 names for the profiles.
What OpenReach call Speed, Standard and Stable, BT Wholesale call Standard, Stable and Super Stable.
BT place customers on either the OpenReach Speed or Standard DLM profile (or Standard and Stable using BT Wholesale terminology).
A number of years ago BT primarily used the Speed profile but use a mix now.
BT will actually change you from Stable to Standard if you ask the right folk on the forum (BT use the Wholesale names for the profiles).
Talktalk do indeed use the middle profile for all customers, or certainly did in the past.
I don't know any residential ISP who uses the bottom profile.
It's used by some business ISP's.