Depending what you wish to discuss its probably just better to wait for the forum team to reply and take the issues up with them. Bear in mind most places run on reduced staff at the moment with Covid-19 so it will take a while for your letter to even be read after being delivered.
Postal writing, while doable, will just get you called up / emailed back and end the postal trail (but still provide a written trail).
Only real reason to postally write at the moment is to cancel.
All that said, the address is as stated in your contract terms:
Hallo Shafreya, I have had a package (BB/TV/Tele) with VM since at least 2010, probably 2007. This contained a V1 set top box which has gradually become obsolete finally dying and being removed from my package in July. My monthly bill of slightly less than £55 remains the same though I have lost a third of my package. It has been put to me that the V1 was over the years, free. Is it likely?