We spent our holidays in Dieppe for several years when our children were younger, and took the Newhaven ferry. Unfortunately Dieppe without a car suffers from being stuck at the end of a branch line from Rouen, which makes getting anywhere else a pain. Boulogne is better connected.Id drive directly to Boulogne (sur-mer) its about 115 miles from Bromley including the chunnel about 3 hours each way. You can get reasonably priced early morning Shuttles and late night returns anytime away from the school break up and last week times. Theres no motorway toll for Calais- Boulogne.
Alternatively if you don't want to take the car go Newhaven-Dieppe. Newhaven Town station is about 200 years walk from the ferry terminal. Dieppe and the baie de la somme towns and villages are at least as good as Boulogne and very friendly and you can ride on the kent & East Sussex railways twin railway. https://www.chemindefer-baiedesomme.fr/en/welcome
I had hoped to get to the Baie de Somme railway on this trip but the weather was too awful to make it worthwhile. But I have digressed a long way from the topic of the thread, which was London to Calais (and connections).