It may, or may not interest you to know, just to set expectations, that the longest known wait on here for a VM install between the first given in writing installation date and when it finally went 'live' was a little over 13 months!
The good news though is that if you have been given an installation date in writing, then VM are obliged to pay you compensation of around £6 per day (as of April 1st, it was around £5.25 per day before then), for each and every day from that install date until they finally connect you up, or you cancel (and yes they still have to pay), or they give up and issue you with a formal cease notice - but even then they have to pay for the next thirty days! VM would naturally prefer to pay this in the form of a 'credit' to your account, but the regulations do not require this and you can insist on a cheque for the full amount.
@baf12345, the same is true for you, except it is not clear in your post if you had VM in your old flat and tried to take the existing services to your new one, or you were simply setting up a brand new account. If the former, then you are probably still paying for the 'existing contract' even though you have no service. In this case, your's would be regarded as a complete loss of service, from the date you move, and that merits a higher rate of compensation, closer to £10 for each and every day!
As you have now discovered, when VM install a new setup for a 'prospective customer' the entire thing is outsourced, which is fine, except that VM appear to have a system, whereby once they hand it off to a contractor, they give up on any sort of oversight. No one working for VM, from the CEO downwards, either knows, or has any ability whatsoever to find out, or indeed appear to have any desire to find out, exactly what is happening with the contractors, when they will do your job, or even if they will do your job!
Just to add insult to injury, if you do call then rather than openly admit this, the knee-jerk reaction appears to be to simply make something up, anything plausible will do, with no ability to base this on what you may or may not have been told previously - because it's all just made up on the spot - how can you simultaneously be at the 'top of the priority list', and also 'there are too many people ahead of you'? Maybe there is a 'super-douper priority list' which comes first? Who can tell, certainly no-one from VM, including alas, the forum team members here, who are, literally, as much in the dark as you are!
So what can you do to expedite things? Alas, very little for the reasons I mention above. What I suggest is that you mentally set yourself a deadline and if VM haven't got you connected up by xyz date, you abandon waiting and sign up with an alternative supplier, even maybe a 4G/5G connection. As regards the VM installation, one option is to cancel and demand the required compensation, no doubt this will be ignored or denied and you end up going to the Ombudsman service. Alternatively, you could do nothing, think about the compensation mounting up and let VM install as and when. Again you demand the compensation but you also have the right to cancel within 14 days without penalty, but you know that the cables are in and tested should you care to give VM another go in the future. A drawback being an unnecessary box on the outside wall of your house, possibly cables strewn across the garden and a couple of holes in the wall!
