The significant difference is that taxes are assessed on an impartial basis - usually a percentage of current market value. The classic problem with landlords is that tenants have no incentive to increase productivity when the landlord captures all of the gains by increasing rent. Even today, businesses often end up in situations where the landlord sees that they're doing well and jacks up the rent in the belief that moving to another location will be too disruptive. My municipal government can demand property tax, but it doesn't hike it every time I get a raise at work.
Referring to 'lord' here is actual lords, like the Lord of Westminster. When I lived there, I paid something like 55 pounds a year into the lords pocket for use of the land.
Landlord in your usage is a different terminology. In the UK a lord with lands either doesn't have the ability, or in practice, does not raise the land rents arbitrarily. Perhaps someone knowledgeable in UK properly law could weigh in.
As other have said, in theory it is different. In practice it isn't. A single property owner has about as much chance of swaying the central government as they have of swaying the earl of whatever.
There is no direct connection between being a landlord and being a Lord - you don't become a Lord by selling someone a leasehold neither are you required to be a Lord to do this (it's just a contract).
Of course, a number of famous landowners are Lords - notably the Duke of Westminster who owns vast amounts of London:
