Steps

Pick the window
Default to May, June & September; write a reason if you choose August.
Read the yellow months on the heatmap first.
Check entry
Electronic travel authorisation is typical; apply before flying.
Check the six-month passport rule in the same sitting.
Pick regions
Two of London, Edinburgh, Cotswolds is enough.
A transfer day is not a sightseeing day.
Lock stays
Lock a London neighbourhood hotel style stay before peak.
For honeymoons, room position beats star count.
Shape transport
Use London-Bath-Cotswolds-Oxford as the spine; cut side quests.
Put the last night near the airport.
Leave slack
Leave a weather/strike/tender day empty.
An empty afternoon is the actual honeymoon luxury.

Entry

Passport — Electronic travel authorisation is typical; apply before flying.
Insurance — Match travel insurance to the dates, including medical evacuation.
Connections — Do not cut LHR / JFK / LAX connections against a night arrival.
Local rules — Drone, temple dress and smoking rules fine faster than a tip.

layers — Pack for United Kingdom diurnal swing and indoor heating/cooling together.
rain shell — Pack for United Kingdom diurnal swing and indoor heating/cooling together.
walking shoes — Pack for United Kingdom diurnal swing and indoor heating/cooling together.
adapter — Pack for United Kingdom diurnal swing and indoor heating/cooling together.
copies — Pack for United Kingdom diurnal swing and indoor heating/cooling together.

Peak as default — Do not pick August just because the month is famous.
Three cities a day — When transfers become the plot, places become a backdrop.
Fake fares — Numbers here are bands, not today’s search-box fare.
Ignoring jet lag — Do not put a dawn shoot the morning after a 7h flight.

Craft does not kill romance. It kills queues.