Travel from Luton Airport. Timing of train reservations

It might be easier to take the service from Luton Airport to Gatwick Airport and board the train to Portsmouth and Southsea there

The travel time of Luton Airport to East Croydon (is the official change point) then East Croydon to Portsmouth is the same as the cross London Transfer via Waterloo. In either event buy a through ticket but you don't get assigned seats on any train.

Note that the through tickets via Waterloo (very attractively priced) don't transfer across London by tube- they are Thameslink to London Bridge, train to Waterloo East, train from Waterloo (Waterloo East and Waterloo are joined stations).

Book tickets from Luton Airport (station code LUA) not Luton Airport Parkway as tickets from LUA include the Luton DART (the train down from the airport to Luton Airport Station).

If the E-gates are working for passport control and if the flight is on time then an hour or so (thus a train after about 1315 (allowing for the DART as well) should be ample.

Buying the cheap Advance Tickets for timed trains off a flight will always be a gamble- you either have too little or too much time.
I would just go for an Open Single at about £56.

Coming back you can and should buy a through Advance Fare from any Isle of Wight station to Waterloo- which includes the Isle of Wight train and the fast ferry from Ryde Pier Head.
Even just for these two journeys buy a Two Together Railcard this week (before the price goes up)- it will be worth it.

Remember that before you reach the UK in May you must have applied for an ETA for each person.

Thank you everyone for your help. I took the advice to purchas the Two Together Railcard.
I will investigate some of the other suggestions.
Susan

I’ve been looking around at the various websites and I’m very confused.
There were suggestions that I buy a flexible ticket for my day of travel. I’m not sure which one.
Here is what I saw today for travel on May 15 for 1 adult for travel from Luton Airport to Portsmouth Southsea.
Super off peak single 50 £;
Off peak day single 62.1 £;
Anytime day single 76£.

I’ve read the explanations of the different types and I don’t understand the difference between super off peak and off peak. I don’t see any indications re: time of travel.

Again, thanks for your help.

Below each fare type you should see the words 'See fare validity'. Click on that-
£76 is valid anytime
£62.10 Off Peak Day Single is valid on any train arriving at a London Terminal Station after 10 am as you will be (no afternoon restriction, at least from Waterloo)
£56 Super Off Peak is valid on any train arriving after at a London Terminal after 2pm (which you may or may not be, depending how on time the flight) and is valid on a train for Portsmouth leaving London before 3.59 pm or after 7.00 pm (you should be leaving before 3.59 unless your flight is really late.

So £56 is a bit of a gamble on your flight being on time. So the sure deal which you know will be valid is the Off Peak Single at £62.10