ok, I'll try and answer seriously:
I don't for one instant believe that visas will be needed for schengen residents to enter the uk ( mostly cos the shengen area would reciprocate in kind, but also because it wasn't that way previous to the borders opening in '92. The Uk never signed or ratified the schengen agreement of 1985 but still didn't need visas there is no reason for that to change) It will be neccessary however to have a valid passport whereas previously a national identity card was sufficient, this is already in place.
The insurance question will deend on brexit negotiations and agreements, there is no real need for this to change, it's just that the uk will have to negotiate with each individual country rather than being able to use a blanket "EU" agreement. but obviously one of the many details that we will need to ;ook into before comitting.
Free mobile roaming... umm... I am old enough, so you certainly bloody welll are! to remember a time before mobile phones existed and I certainly don't need one, if the cost is prohibitive then we'll manage without.
as for why they want to do a uk trip, the reasons are various;
There is a certain fascination within the french psyche with Britain, and all things British. we are reputed to be a little odd, quirky, individualist, not to say bizarre and god forbid, gentlemanly! these are obviously stereotypes but the idea is there. This particular group find themselves with the possibility of exploring a foreign land in the company of a native, who will help them, or so they believe, avoid any "faux pas" or communication problems, who is used to riding on the wrong side of the road and all the other anomalies that make us "so British".
the French are, on the whole not a very adventurous people, and don't need to be as they have beaches and mountains galore in their own country and if they feel the urge to go abroard then french polynesia or Mayotte enable them to get exoctic whilst remaing at home within their own administrative framework. Posibly more importantly thir language is not universal, where ours is.
The pyrenees belong to them, as do the alps, the jura, the massif centrale etc etc and have already been done forwards, backwards and inside out. They have no interest whatsoever in going into Germany ( forgiven yes, but not forgotten...) other than to have a go on the autobahn. I have singularly failed to spark any interest in a run to the nurburgring for example.
dunno if that makes sense