Seems over-utilisation is the cause.
The BQM you posted shows a typical over-utilisation pattern, meaning more traffic than VM's local network can handle. The particular giveaway is the way that latency magically improves at half past midnight, and then builds again through working hours to a worst case usually in the 7pm-11pm slot when the network it typically busiest.
Nothing you can do to improve matters. In some areas VM undertake work to rejig the local networks to balance loads and eliminate over-utilisation. But sometimes/often that's either not possible, or judged uneconomic if there's a need to spend money on more equipment. And sadly VM won't ever admit the truth, so even where there is a fault reference and a "fix date", but there's no way of knowing if that fix date is actually backed by an actual plan of action and programme of works. Quite often it seems quoted fix dates are simple unabashed lies, and as the fix date approaches it is simply moved a month or two ahead. It is possible your area will be in line for improvement, but there's no way of finding out. So, if the problem persists you have two simple options:
1) Sit it out, and hope that either VM do carry out improvement works. There's little or nothing you can do to force VM to upgrade the network, nor to be honest about the outlook. This has been the case for years now, although the actual incidence of over-utilisation has diminished over time as VM have fixed the worst problems (no help to you, of course).
2) Get yourself a new ISP. If you're in a fixed term contract you'll probably have to use the VM complaints process (and almost certainly escalate for arbitration at CISAS ) to be released from contract without penalty. If you need to do this, the grounds of your complaint is the poor performance, and your request fro release from contract without penalty is twofold: First the Consumer Rights Act 2015 that requires any consumer service to be provided with "reasonable skill and care", and second, the Ofcom Fairness Commitments, that state "Customers’ services work as promised, reliably over time. If things go wrong providers give a prompt response to fix problems and take appropriate action to help their customers, which may include providing compensation where relevant. If providers can’t fix problems with core services they have promised to deliver within a reasonable period, customers can walk away from their contract with no penalty."
