As I've always understood it (rightly or wrongly) your existing line at the original cabinet, will be linked to the fibre in the new cabinet via copper.
So assuming that the they haven't found a cable joint between copper and aluminium, in one of those water filled holes (that's not quite clear from your post), some questions you need answering are:
1. Did they know your line was aluminium at the point of planning the cabinet installation.
2. Are you the only aluminimium line connected to that cabinet, and if yes have others complained.
3. Is it current policy to connect aluminium cabling to the FTTC service, for minimum improvement.
4. If not, why did the engineer not notice the aluminium at the point of your FTTC installation.
5. Is the line to your house ducted.
There's been much talk about aluminium performance with VDSL, but many would claim that the condition of the line to be more important over short distance, than the cable construction, and you already mention water filled holes etc. If you had a quality problem with your phone line, you would get the fault fixed or a new drop put in. So it's not impossible to force Openreach to act, but your ISP will have to be the ones to enforce it, otherwise you'll be continually be quoted the minimum acceptable speeds of an "UP TO" service.
My advice would be to see if any of your neighbours have similar issues, and push your complaints to someone nearer the top of BT directly. Going through the support functions will likely never get you to a satisfactory conclusion.