Alex, there is a prohibition on operating an aircraft near public gatherings in the Air Navigation Order, CAP393. Glastonbury in particular is usually NOTAM'd to remind folk about this.
I think the operative word is "
near but the O.P. was asking about "dropping in".......I would suggest that this is possible, subject to prior permission and possibly the organisation providing a proper landing-area etc.
If you dispute this, you also infer that "public gatherings" such as the F1 Grand-prix ,this weekend at Silverstone and the Goodwood Revival weekind,among many others, all allow huge illegal air-traffic movements.
This is patently not the case, therefore,the answer should be,-
"Yes, subject to permissions and legal compliances."