Advice needed

Are you sure your problems are not Wi-Fi related more than max speed related ?

Using a Wi-Fi Mesh especially in a multi hop fashion looses speed and add latency and jitter at every hop.  Latency and jitter appear to degrade MS Teams video calls far more than any other conf platform.

Assume you are using the "Powerline" to connect the Mesh?  Best to connect via ethernet for the backhaul.  Using mains wiring is not a good idea, especially as it's an old house.  Ignore the marketing, PLA's don't perform as marked on the box.

As noted by Client62​  Teams will also benefit from a cabled connection, whether it's with BT or VM.  As for your Son, again cabled is the way to go for gaming.

Have a think about running cables externally to the roof space, for example, and using drop cables into bedrooms etc.  Cables do not always have to be run internally.

Hi Fluorimeter​ 

The extra TV boxes do not provide a mesh facility - they don't work the same way as Sky Q boxes.  However because you will be subscribing to a Volt package then VM will provide up to three wifi pods as part of their wifi-max and wifi speed guarantee.

This WiFi guarantee is VM's promise that you’ll enjoy download speeds of at least 30Mbps in every room.  You can only order 1 wifi pod at a time once it's all up and running.  This minimum 30Mbps speed in every room may not make any difference to your fathers large victorian house, and will need very careful placement.

As to how to order the extra TV boxes, I've provided a complete walk through of the order process below, including a screen shot of the second page of the order process where you have the option to customise your bundle.

On the website select the Mega Volt bundle
On the next screen click on "Customise bundle"
On the next screen click on the speed you would like, and if available in your area it will also offer the option to select an upload speed add-on which gives you an upload speed as fast as your download speed for an additional £6 per month (this is only available on the full fibre packages with speeds up to 2 Gig)
Then select Continue which loads a screen allowing you add up to 2 extra sims
The next screen offers a selection of additional add ons (TNT Sports, Premier Sports, and Racing TV.  Select any of those you want added then click on Continue
On this next screen yo can choose a different Netflix tier, and also add Disney +
Then click continue, and on this screen you can add up to five additional Stream boxes.  The first addional box is an extra £10 per month, and any further additional boxes are each £5 a month extra (so in your case you would add an extra 3 boxes for an additional £20 per month in total, giving  you the 4 boxes you require)
The next sceen ask if you have a PAC or STAC from your current mobile provider which you should choose if you want to keep the same mobile number
Then after clicking Continue you are able to book your engineer appointment

If you go for VM, put the hub into modem mode for the Decos to work properly. Also use Ethernet cables to connect all the Decos by Ethernet cable as wired backhaul trumps wireless every time. Here's how you do it: hub to main Deco and small unmanaged Gigabit switch to Deco, now remaining three Decos to the switch. If necessary hire an electrician to do the drilling to run the cables. For each Stream box run an Ethernet cable to the nearest Deco Ethernet port or switch.

Hey Fluormieter, thank you for reaching out and for your question today. We hope some of the advice has helped.

However we would advise to contact our sales team on 0800 183 1234 they can put extra boxes onto the order.