Why do we need a trial?
It is a market trial, not a technology one. The reasoning then gets convoluted.
10 years ago, when NGA was being discussed before BT were allowed to roll anything out, every provider was keen to emphasise that being tech-neutral was an important feature. Fair competition of any technology was important (including mobile & wireless).
Now, however, the government is seemingly embarrassed about the UK's position on the fibre powerpoints. They, through Ofcom, have changed to pushing full fibre without any attempt to keep that neutrality. The arguments why fibre is a necessity, in the face of (say) DOCSIS 3.1 and G.Fast have been noticeable by their absence.
Ofcom just set the new rules for competition moving forward, incorporating the aim of new companies bring full fibre through PIA. They want 40% of the country covered by someone who isn't Openreach or VM.
This money is the government's carrot for getting companies to start in this market, and isn't for BT or VM. Once companies have started, the hope will be that it triggers further self-funding to move forward; perhaps by getting a company over the "startup cost" of entry, and perhaps by helping to show the demand via takeup.
But the government isn't allowed to affect the market through state aid - and BT & VM could challenge legally - so the "trial" becomes a label to subvert that.
Edit: Remember the "market trials" in BDUK? Where the government funded some trials for wireless and satellite, trying to get some more competitors involved. All hailed at the times as greatly successful, but it hasn't actually spurred that much wireless/satellite in BDUK as yet.