Rail card before purchasing rail tickets?

You can purchase your tickets prior to purchasing your railcard(s) and state that you have a railcard during the purchasing process. You must purchase the railcard prior to the first trip. Will you always be traveling together? Would a "Two Together" Railcard work for you as well? It might save you 30 pounds.

The problem is that we will need to leave early (8:30 a.m.) on the two longer journeys, and I think the 2-together card is not good for early departures. I hope I am wrong!

Ah, I think you are correct. Two senior cards it is then.

regarding timing - it boils down to your future plans.

They are valid for a full year. If you will be back before they expire then I'd suggest getting them at the last minute. You can still buy ( Advance and other ) tickets with the discount as long as you have the Railcards before you get on the train. That stretches the time period to the max.

On the other hand if it will be over a year before you return to our fair land (boo hiss) then having the procedure done early lets you check a box and one less thing to worry over and do.

Whichever works for you. Just be sure you have the Railcards before the journey using them.

I think the 2-together card is not good for early departures. I hope I am wrong!

Checking the website for the two together card it does state quite clearly
A Two Together Railcard can be used to buy discounted tickets for travel from 09:30 Mondays to Fridays, and at any time on Saturdays, Sundays and Public Holidays. The only time you can't use your Two Together Railcard is at morning peak times, i.e. before 09:30 Monday to Friday.

Also note that the sernior railcard can be used on discounted over 60's tickets for travel across the National Rail network at any time of day – except for journeys within the London and South East area during morning Peak hours – and at any time on Saturdays, Sundays and Public Holidays.

Between London and Canterbury, you will have to leave AFTER 9.30am

Looking back at your other post, for the Canterbury to Chester journey there is a route specific relaxation of the morning peak rules on the High Speed route to St Pancras which allows you to use the 0921 from Canterbury on a Senior Railcard.

Thank you all for your tips. In a few days I will be able to decide what I will do.

You only need to have the rail card when travelling - so buy now and tick the box saying you have it, get the card later.

Of course, that's only worth doing if you are coming back and want to maximise the efficacy of the 12 months duration of the card