Since no date had been set for the visit, how can the date be changed or put on hold?
Probably because it has been known for awhile now that Trump was planning a visit there. It has been a matter of controversy over in the UK. It's not been disputed that he planned on going. Whether or not they had a time frame in mind I don't know but it was planned for him to go, reported in the media and the UK never disputed it. Many times the exact dates and schedules are not released until right before the trip for security reasons.
I hardly think that Britain would "disinvite" Trump just for spite but I could see a valid security concern both for the UK and for Trump. There is a lot of turmoil over there right now. And Trump does have a reputation for stirring the pot:(