You should have the choice of all ISP's who use BT Wholesale to provide their services (i..e carry their traffic) - the vast majority of ISPs.. most of who do not do location based charging.
Unfortunately that isn't the case.
A growing number of ISPs no longer provide any service at all for 20CN exchanges - which are commonly Market A.
Even those that don't do location based charging do charge a higher rate for 20CN - which is effectively the same thing as 20CN and Market A tend to be closely related.
For example, Sky will resell BT in non-LLU areas, and have a free broadband offer - unless you're on a 20CN exchange in which case they charge £17 pm. And they don't support ADSLMax Premium at any price.
I've been through almost the entire list at ISPReview, and there are no ISPs that are more than £1/month cheaper than Plusnet for ADSLMax Premium, and most are significantly more expensive if you want a fixed IP address and unlimited downloads.
And disregarding price there is no choice that would affect the quality of the connection as the backhaul is the same for all ISPs - there is effectively no choice.