To your first question, if you are asking whether it is good for Britain for us to be producing (some of) our own food, then surely it's doesn't matter if that is being done by landowners or their tenants? The land is still being cultivated.
The second question is more interesting. It seems to me there are many categories of person out there who could class as farmers. There are landowners who actively cultivate, those who directly employ someone to work the land (i.e. a farm manager), those who use a contractor. Then there are tenants and landlords. Then there are all sorts of shades in between. I know one guy who owns a smallholding, leasing a few paddocks to horse owners, running sheep over the rest and renting much larger blocks from other landowners for more sheep, while also doing farm related tasks for other people (relief milking, lambing help, etc.)
I know there are some who like to vilify 'hobby' farmers, but how many out there can honestly say their family could live off the produce of their land once they've subtracted any other income (including sfp)?