Generally it is considered infra-dig to ride a perm route that overlaps the calendar event too closely. I'm not sure whether anybody thinks a group perm to York at Easter fits into that category.
I think commenting this and then implicitly sitting on the fence is 'stirring it'. The Easter Arrow is an esoteric calendar event and the reasons that mirroring a calendar event with a contemporaneous DIY might be considered 'infra-dig' don't (to me) seem to apply. Geographically spread start locations and times; different routes (with pleasant intersections, hopefully, with those riding from a similar directions) mean that those riding a DIY 12 hours ahead (say) will have no adverse effect that I can imagine. Go for it - sorry not to see you in York on Saturday morning, but riders' real lives are important to a majority/minority(?).
if the laid-down start window is enforced (for 'rules adherence', reasons shared above or otherwise) - I asked about this on here 7 weeks ago (hence I posted the quote from the ACME thread which gave the answer) - then a Thu evening to Fri evening DIY offers an excellent opportunity for a ride, towards York, or towards anywhere, or a loop.
Someone else mentioned Hull Trains. Looks like the London train from Selby (from York = 22km) will cost about £14 (to King's Cross) if booked late this month.