Do you have insect.pest exterminator companies in the UK? The kind that
come in and apply professional level chemicals for insect control?
There are compounds that not only kill the adult insects, but prevent
the larvae and eggs from developing further or sterilizes them.....
Much more effective than chemicals we can buy in the stores.... This
would be the approach I'd take, if it's available.

Otherwise, I'd use the boric acid sprinkled into the rugs /carpets, and
along base boards and on throw rugs in rooms without carpeting. Brush
in with a broom.

On the same day the house is treated, I'd treat the car with a spray or
the boric acid, and treat the garden with a spray on chemical or
granules to reduce the fleas carried inside.

Jo Wolf
Martinez, Georgia