Attached to the leaf of an English Oak.
No interactions present.
This looks more like a currant gall, caused by the gall wasp Neuroterus quercusbaccarum, especially as it was on a leaf, rather than a stem or bud.
The time of year (early June) rules out other rounded galls.
Lat/Lng: 55.2427, -1.6231
OS grid ref: NZ240943