Do they pay the boarding fee to get the place and then not use the boarding facility then?
I believe they have to use the boarding facility - the closer school is the one I know about and I would imagine they have residency rules for the boarders like most boarding schools would - weekly boarding would be possible I'd think.
The only family I know personally who did this also had the constraint of a possible future overseas posting for Dad (which had happened by the time I met them, but they chose the boarding at the outset just in case), but were at one point living locally with at least one child boarding locally - weekdays only I believe. They told me about other families who lived locally enough not to board, with no such issues, who still had a boarding place.
Of course such schools are generally pretty good, though, so they have a lot of non-local children paying for the boarding place to get a good secondary school place at a fraction of the cost of private school, as well.