Ordering 2-Together Train cards in advance/Train on bank holiday

Don’t bother with the digital card. First buy tickets indicating you have the card, then when you arrive in the UK buy the normal card at a train station before you travel.

https://www.twotogether-railcard.co.uk/download/clientfiles/files/RDG_Two_Together_A4_Print-at-Home_Form_JB_6_0.pdf

There is major work at Euston over the bank holiday – yes schedules are affected with reduced services so allow extra time

http://www.nationalrail.co.uk/service_disruptions/currentAndFuture.aspx?TravelDate=27%2F05%2F2019&TOC=vt

You do not need to have the Two Together Card in hand when you buy train tickets online using the discount. Just buy it when you arrive. That is what we do.

I've actually booked all of our trains as though I had the Rail Card in hand. I was just trying to eliminate, if possible, the extra step of buying them when we arrive and are jet-lagged.
Thank you for the link about the delays at Euston. I was unable to find this myself and really appreciate it.

what time do you need to be at Watford Junction on the Bank Holiday Monday?

We have 10am tickets to Harry Potter, and they would like us there by 9:40am. We'll take a short bus ride from Watford Junction to HP, so working backwards, I would guess we need to be at Watford Junction by 9:30am.

Don't count on the bus running as frequently on a bank holiday - I certainly wouldn't. The last train I would be comfortable with, since there is such disruption due to the boondoggle known as HS2 which is a major disrupter these days at Euston and especially on Bank Holidays, is the 8:47 London Northwestern (shown in the timetables as West Midlands Trains who are the operator of record, they are the same train) service to Crewe, stopping at Harrow & Wealdstone and the next stop is at Watford Junction at 9:05.

Any later and you are at risk.

The 8:28 London Overground train is an all-stopper and I don't recommend that.

The previous fast train is the 8:21 London Northwestern service to Liverpool Lime Street arriving at 8:40, having previously called at Harrow & Wealdstone and Bushey.

So much can go tits up on a Bank Holiday Monday I would wear a belt with my braces.

It is very little trouble to buy the pass on arrival. Here is how you can speed up the process. Have your photographs ready in advance and complete the application form in advance. The agent will also print out your tickets for you so have your confirmation numbers organized and available.

Thanks to all of you for taking the time to advise me.
Nigel, I had done some research (which was so much more useful bc of the info you shared with me on my first train question several weeks ago-Thanks!) and booked anytime tickets, with reserved seats on the 8:47am London Northwestern train. I had wanted flexibility in case we needed to leave earlier, and then as well for the return. Fortunately this is a short trip, and therefore the anytime wasn't too expensive. I appreciate the confirmation that I had planned the timing well, because I just don't have experience with it. The bus I was mentioning is actually the dedicated Warner Brothers HP shuttle bus, so I am thinking that since everyone using it will have reservations, it should continue to run on it's usual schedule.

Everyone, Thanks for the advice on the train passes. I really like to get the details right and sewn up as much as possible, but you have given me the confidence that this will not be a big deal to do upon arrival, provided we are prepared.
I enjoy doing my own work, and learning for myself how things work, but this forum has really been a gift to me in sorting out a few problem areas - I really appreciate everyone who responds and takes the time to help us have smoother trips!

well done - and good to have sorted.

For the benefit of others in the future, on a Bank Holiday, Anytime tickets are overkill - it is a completely off-peak day. When you are looking for your seats, just take some together. London Northwestern doesn't number seats and doesn't offer reservations in either class.

Because of the disruption and the bank holiday except for overnight drunks and people going to participation sports expect the early morning trains to be empty, and the later afternoon ones quite crowded.