Hi there and welcome to the forum
It looks like the BT profile appears to have got stuck... (the bRAS and IP profile are the same thing), due to the result you've got from the BT Speedtest. This is completely independent of ISP, so cannot be traffic management.
Assuming you have constantly synced at 2.9 or thereabouts for the last 3 days, your IP profile should be 2500 or maybe 2000, but certainly not 512k.
Try rebooting your router once a day, which has been known to kick the profile update into life in the past.
Failing this, if nothing happens within the next 3 days, then the only people who can fix this are BT Wholesale, and the only people that can ask them to do so are your ISP. So unfortunately you need to somehow make Pipex understand that the problem now lies with your BT IP profile which is stuck at 512k. They should then be able to raise this as a fault to BT who will unstick it.
Migrating probably won't solve the issue either. All migration does is to let you log onto a different ISP using a different realm (the bit after the "@" in your logon name), it doesn't, as far as I know, trigger any training period or anything with the exchange.
Ring pipex back and tell them that sending a BT engineer is useless, but it still needs to be raised to BT to get them to sort out the stuck bRAS profile.
Hope this helps
Chris