In the UK you can (and usually do) unionize without a workplace.

Many are industry-specific such as the "Communications wokers' union", but there are also general workers' unions such as GMB [1] or Unite.

It would be possible, indeed probably preferable, to form a "Delivery workers' union". It would be a union of delivery drivers who would pool resources to fight for common rights.

[1] https://www.gmb.org.uk/campaigns/deliveroo/