I've used Whitby slip a couple of times in the last month. I've found it easy to launch with just 2 people. The ramp is wide, sufficiently wide to launch/ recover two boats at the same time.
There's ample parking for car and trailer, although when the weather's good it does get busy so best to get there early to get a parking spot. I paid just less than £18 to launch my RIB and park my car and trailer last Sunday.
The marina office opens at 8:00 AM. The chap who runs the office sometimes leaves the office unattended to carry out duties around the marina, though he's usually only away for a short while. I've found them to be very helpful. If weather conditions are marginal, he'll advise you to take a walk along the pier to see conditions at the harbour mouth, before you commit to paying your launching and parking fee.