Hi Alpha
Here my thoughts, based on my experience of concerts.
What to expect???
Expect a nice concert and nothing else

Last year a French newcomer came to Arundel, expecting a lot of things unrealistic, like chat and talks with the boys, contact, handshake, video recording and so. He had "only" a very nice concert, on a very well located seat, and despite this he felt deeply frustrated, causing him crisis and depression.
this person is an extreme case and is not representative fortunately, but this is the message : to be short and dry, we will sit, they will sing, we will leave.
Having a brilliant concert is what you have to expect
Any hints and tips for a first timer??
The main point : don't arrive too late at the queue. Better for you to loose 2 or 3 hours in a small queue rather than to save 2 or 3 hours and find yourself seated far in the back of the church.
Have with you a folding umbrella, a little snack.
Be prepared to talk and exchange with unknown people around you
What to wear, what not to wear??
Just dress in a nice and comfortable way for you

Some (a lot) are in jeans, some wears a jacket (a few), almost none wears a tie
Camera?? Photos during the performance?? (That has to be a No??)
Absolutely no photo is allowed during the concert. A few people use the vague moment at the very end, during the bowings, to take a few photos, but this is the max you may do.
of course before the concert you can take a lot of photo of the venue, the building, inside, outside, the empty stage...
Is there usually a programme?? Does it cost???
Yes, there's always a programme.
A few years ago it could have been a simple A4 B&W photocopied paper sheet, nowadays it's more a 8-12 A4 pages libretto, full color

Price is usually cheap in UK 2-3 pounds. it's possible they also sell pre-signed programs at price £5 or £10 (?)
Meet and greet???
No chance at all. There was no M&G in UK since 2008. Don't expect anything because there will be nothing.
Will the cathedral be open during the day? Will there be tech rehearsals??
Usually the church on a concert day remain open to the public until 15:00, 15h30 or 16h00 at max.
before that you are free to walk in and see the technical setup, lights, sounds... be careful not to disturb the techs

When the choir will arrive, the church will closed and the queue will start to really forms. The boys will enter by a side or back door and you will not see them. All the choir rehearsals are done with doors closed, you will see nothing and hear almost nothing
Is there anything else that could enhance the day/evening.
Depending of the announced weather, dress yourself in adequate way to bear the cold, the rain, the sun....
Check 5 times that you have not forgotten your ticket at home

if you start the queue early, which I encourage you to do, be prepared to have no diner till the end of the concert, so a snack in a backpack is a good idea.
If you have one, try to have with you a small cardboard tube, if there's any poster sold at a desk at intermission.