Your "Point 2" is the problem. Pure and simple.
Post Office broadband is completely run by TalkTalk. In most cases of course, it will be on their LLU which isn't too bad, though how good the supply to the Post Office is another question.
Because your exchange doesn't have TalkTalk LLU then you are on the BT Wholesale network. However, ISPs using that network have to rent handover capacity. That is to say, a maximum Mbps they wish to pay for, shared out between the customers they have on it.
TalkTalk non-LLU has always had a dreadful reputation for woeful under-provision, as it costs a lot more than their LLU. (Which is why LLU providers can undercut all the BT Wholesale ISPs). The non-LLU Sky Connect was as bad.
You should be fine with most non-LLU ISPs. One very attractive one will be Plusnet, but watch out!
It's an 18-month contract but 12-months free. As it is not in their low-cost area the basic price after the twelve months will be £17.49pm, not the advertised £9.99pm. Line rental and Anytime calls are both going up in September. I'll leave you to do the arithmetic to see what your 18-month charges will be and average it out. Their Customer Support isn't having a good time at the moment.
You should also take a look at this post
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
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