Traveling from London to Normandy

Forget the ferry option since the weather could messed that up.
Since you have your lodging already reserved in what towns are you staying in?
That information could help solve your car rental dilemma.
There is limited service from London Southend Airport to Caen; that airport is about 45 minutes from London.
Otherwise take the train from London to Paris and then to Normandy

Recent threads on nearly the same question:

From the 8th of March:
https://community.ricksteves.com/travel-forum/transportation/london-to-normandy-99b432de-bdb5-4903-b8a8-18e73ae7f0ef

From the end of February:
https://community.ricksteves.com/travel-forum/france/travel-from-bayeux-to-london

From the end of January:
https://community.ricksteves.com/travel-forum/general-europe/getting-from-london-to-normandy

From the beginning of January:
https://community.ricksteves.com/travel-forum/general-europe/london-paris-and-normandy-for-7-9-days

Perhaps these will give you something to start with...

Forget the ferry option since the weather could messed that up.

With the commemorations in June it is highly unlikely that there will be weather conditions serious enough to force a cancellation. I've used the cross channel ferry in some very bad conditions, especially in the winter months and they've continued to sail. Also, from my house I can see the ferries departing and arriving and it has to be seriously bad conditions for them to be cancelled, something very unlikely in June.

That being said it's not the most expedient way to travel however if departing from Portsmouth the OP can visit the D-Day museum and perhaps Southwick House where the invasion was planned before the ferry journey which will take you directly to Normandy.

We're staying in Vierville sur Mar in Normandy, but everything is so close there flying into Rouen or Cherbourg would be fine.
Thanks for supplying those threads, i thought I searched adequately but apparently not! I'll check those out.

The only airport open to the public in the Normandy is Caen.
Train to Bayeux rent the car there and drive to Vierville sur mer.
Drop the car in Bayeux train to Paris then back to London\

Hi we are leaving for London and Normandy on Sunday.@after much discussion we’ve decided to abandon the idea of beginning Paris or even visiting as we’ve already been there. Instead we are taking the Eurostar from London to Lille Fr and renting a car, staying in Bayeaux for two nights...might be a good suggestion...

wendy keller, be prepared for the high toll on the Normandy Bridge. You'll have your IDP I guess?

The rental counters are upstairs at one end of the concourse. The rental garages are up there too.

If you're staying at the Hotel Logis Casino in Vierville sur Mer, I can vouch for it being very nice. Our room was spotless and comfortable, and we loved the breakfast room with huge picture windows overlooking the channel. The croissants and all the rest of the breakfast items were fabulous.

To drive from there to the American Cemetery takes about half an hour without traffic. It's a short distance, but the road is not straight. Since you will be there on the D-Day anniversary you can assume there will be quite a lot of traffic throughout the area, and parking lots may fill up early.

One alternative is to fly from London Southend to Caen direct on Flybe and rent your car at the Caen airport. After your Normandy trip, you can drop it off in Rouen and take the train to Paris. When you're searching for car rentals, consider AutoEurope, which often has the best rates.