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They are definitely not frogspawn and frogs need to lay their eggs in water. In the rare cases when frogs do lay out of the water you would see the black eggs without much jelly around them.
The frog had almost certainly eaten the slug and been startled by you getting the camera out.
As these aren't amphibian egg, I suggest you move it to the invertebrate section using the edit tab at the top of the observation.
Lat/Lng: 51.0734, -1.8714
OS grid ref: SU091304