The question of is often depends on the water authority. As a general rule it is not a good idea as a storm could and does put undue pressure on the foul drainage with any flooding having particularly nasty and unpleasant consequences.
Whether it is legal to connect the storm to the foul I don't know.
I have never come across this in new build construction until now.
The NHBC Standards state:
"Where the sewerage undertaker permits surface water drains to be connected to a foul water system: an interceptor should be installed on the surface water side of the foul sewer junction, or trapped gullies should be used."
Most drainage should be greater than 450mm below finished ground level. If it is any shallower, it should be protected with a concrete capping.
In your situation, I would look to connect the RWP outlet to the nearest storm drain run which should be below your proposed patio formation level.
More worrying, is the fact looking at the brickwork that originally, the airbrick was below ground level and I suspect your brickwork DPC is on the top of the airbrick?
The dpc level must be 150mm above finished ground level.