Yes handover capacity is the BIG one, BT Wholesale is more expensive than TalkTalk LLU for that, so tends to be under capacity for TalkTalk based services.

Of the large 5 its really just BT, or maybe PlusNet, then into the Zen, IDNet arena and more SME pricing.

It does cost BT Retail more for your custom on those exchanges, just they average out across the UK.

Yes

I would agree with CecilWard. It is certainly galling that those of us on long lines have to pay more for our broadband, but we have to make the best of a bad deal. Andrews and Arnold will offer you excellent backhaul and line stability from their end. When things do go wrong between you and the exchange they have excellent customer service to sort things out and give you the best available speed. Generally this is true of most of the smaller internet service providers but you have to pay more.

In reply to a post by CecilWard:

Take a look at aa.net.uk, the best of the best. They will sort out your speed problems for you. You won't regret it.

The OP hasn't got a speed problem as such. He has TalkTalk congestion. It's a known problem on their non-LLU lines, as a few of us have said.