Your contract is with the ISP
"Legally your ISP is still responsible but they don't always have much control over the last mile."
That's a contractual issue between the ISP and Openreach - and is explicitly not your or my legal concern.
If you want to get out of a contract for non-performance (particularly if throughput is shite or there are lots of disconnections), send a recorded delivery letter stating that the performance is not as advertised and therefore is "unfit for the purpose it was sold" - this is a contract breaker. They have a short period to try and remedy the situation, but after that you can close your account without penalty. (Disclaimer: I am not a lawyer, but this is the advice I got from one). They may try to go after you for termination fees, but proof of delivery of the letter is a powerful tool in small claims court.
You are NOT bound by Otelo or any of the dispute processes - they can be leapfrogged and small claims actions taken.
You can also invoke the Protection from Harrassment Act 1997 if they're making a real nuisance of themselves.
On the actual problems side:
Some ISPs are far better than others about badgering Openreach to actually fix duff lines. (Mine is pretty good - http://phone.coop), whilst some (ie, the big 6) seem to have some kind of cozy agreement with openreach to not bother and then get their ScriptReading Monkeys to divert/distract/delay.
This is especially important when there's a faulty cable involved. $LARGE ISPs are utterly focussed on "closure rates" and not on patterns of complaints, which means they don't put 2 and 2 together when a large number of customers in the same street are complaining about service issues (or when a single customer is repeatedly complaining about the same problems). BTOR contractors will happily sign off a non-fixed fault as "closed" because they're focussed on closure rates (they should be flagging patterns of fault reports, but they don't, because it might mean they have to spend money.)
A smaller ISP is often willing to keep badgering BT to fix the bloody fault - and there's none of this "you have to wait 2 weeks" bullshit.
My personal experience of cable faults is that it took 8 _years_ of complaining before openreach finally worked out that the underground 7 pair cable into my property was shagged - despite every contractor coming out finding foreign battery, shorted pairs, etc etc and changing to the "least bad" pair over that period - and 4 months for them to actually get around to replacing it.