Not necessarily shygirl, I have to check at least twice a day for fieldkept ponies and more when they first go in to check everything is settling ok, or if something is looking iffy. But I rent by the field not the head, put a few over and take them out pretty prompt, ask before I do work to land/fences, they're not shod or ridden as they're breeding stock or youngstock with OAP nannies to keep things calm for the most part - so I don't see myself as more hassle than any other livestock owner. And I imagine there would be smallholder/farmer type tenants that would be there all day if they'd rented it to be their smallholding and wanted to spend time enjoying and working and taking their kids down at weekends/holidays too.. Just saying, not all horsey folk are bad tenants or even riders..
Anyway, the original question, I think licenses are optional depending on the services you offer, to let a field for grazing you don't need one. If you have stables, a yard, multi-owner livery arrangements, comings and goings with kiddies and pets and your machinery and public events, shows etc, it's a different matter and needs insurance to cover it at least, but not a field for grazing only. Don't put jumps in it tho, or anything that suggests it is being used for riding in, that's planning disaster as they'll look at you for change of use and all kinds of hassle.