Mr Max, don't you have Vivid 350 from Virgin Media now? Why the worry about a couple of megabits here and there with the VDSL?
Permitting customers to start playing with target SNR margins is a bad idea from the Openreach point of view. Given the stories we've seen here of people degrading their services mucking around it would go against the entire point of DLM, to reduce manual intervention and faults, to allow end users to manipulate it.
VDSL is going to be receiving fewer and fewer interventions from Openreach. It is legacy technology now, has been obsolete for a while and the level of investment going into it is minimal.
Regarding your cabinet the chances are there won't be a second one built but either additional line cards / tie pairs to allow use of 5th and 6th cards or an HD upgrade to move it from 288 lines to 384 capacity. By the end of the month is probably too soon for an entirely new cabinet, the delivery times for those are of the order of several months. Be aware that if there is any room for further impact this upgrade is likely to increase crosstalk.