For the vast majority of installations, VM use their own ducts and infrastructure, so chances are the existing ducting under your garden won't be a problem. VM normally do very shallow cable lays, so any expectations you have about a properly buried duct may be optimistic, and rather than ducting it's likely to be 30mm ribbed conduit. Of course, they may use their patented ductless installation technique.
In terms of service quality, it's very variable. It's good for most people, most of the time. My own connection was mostly very good for over 20 years, but since the beginning of this year reliability has been appalling (and with VM's poor customer service and even worse complaints team there's not much can be done to drive improvement). VM's infrastructure uses the ageing DOCSIS technology, and whilst it's a marvel of ingenuity, it's not the way you'd build a network now, and is prone to a range of difficult to trace problems, which if they manifest are most harmful to video-conference types of application (eg Teams) and gaming.
In your shoes I'd stick with BT and negotiate a discount if price is a problem, but your other option is to order VM, see how quickly and how well it is installed and works, and cancel in the 14 day cooling off period if it is less than perfect? Bear in mind that in eighteen months the VM price will double as your discount expires, and you're back to square one.
