The process that they are, trialling in the UK uses between 3-10kg of plastic per ton of asphalt. The total UK production of asphalt is around 25Mt. So if this process was applied to all asphalt production, it would use a maximum of 0.252Mt of plastic. The UK uses 5Mt of plastic per year. So this can account for a maximum 5% of the total UK plastic consumption.
5% is a small but non-negligible contribution. Especially if the asphalt is actually stronger and more durable with plastic added.