On the face of it , it seems that your current MSG is 38Mb , your renewal ( should you re contract ) will go up to 43Mb , probably because for all the months your existing contract has been running , your actual connection speed etc has been observed, so there is now real data from your connection to base your ‘new’ MSG on , ( rather than a mathematical calculation that was used to give a MSG before you actually became a BT customer ) if you don’t renew and continue with your current deal the MSG is 38Mb , if you renew you get the ‘benefit’ of the higher 43Mb MSG , but in any case your actual speed is 30% higher than even the ‘new’ MSG if you re-contract anyway.
For all your apparent problems, the actual observation of your line data over the previous period has deemed it’s actually better than before and the MSG has been raised accordingly,
if you have a ‘hard’ fault ( if , as you claim , when it’s raining the speed drops ) any ‘physical’ issues like high resistance, partial s/c , earth contacts , battery contacts etc , on the line can be detected with a line test , so even if the speed hasn’t dropped to below the MSG , these defects are enough to get OR on the case , there is no barrier to BT calling OR out when a faulty line is detected.
If , as you state the speed drops but not as far as the MSG , and in your experience, BT won’t raise a fault with OR , simply report the issue differently, don’t simply say ‘my speed occasionally drops from 57Mb ( which is what the router stats show ) to below 38Mb but return to 57Mb, but as you have had OR visits already , presumably some of your fault reports have been progressed to OR
TBH , if this instability was frequent, it doesn’t really explain why you line hasn’t been banded due to this instability .
