Brit Rail questions

You need to work out the cost of the Railcard v doing the trips with point to point tickets. You will be able to fond the cost of point to point tickets here:> www.nationalrail.co.uk
Now, on most short journeys - the price is the same - whether you pay on the day or pre-book ahead - so I would pay on the day for these. On longer distance trips - such as Exeter to Cheltenham and to & from York, you will find it is cheaper to pre-book ahead a specific train and the cheapest fares get loaded around 11 weeks out. (The price gradually rises though even buying the day before will save money. Go on the aforementioned site and play around with dates and you will see what I mean).

Also consider buying one of the Railcards but note it is only the Senior one that is valid before 9.30am on weekdays. If 2 of you are travelling - the 2 together Railcard for £30 works out cheaper.https://www.railcard.co.uk

"Stansted to Paddington" - trains from Stansted airport go to London Liverpool Street station (or north to Cambridge etc.). To get to Paddington you need to get the tube (London Underground), which, AFAIK, is not covered by a Brit Rail pass.
Is your tour pick-up at Paddington station?
York to Stansted is two trains: York to Peterborough and Peterborough to Stansted. The second train stops at Cambridge, but you don't need to get off there if you don't want to.

Can a Brit Rail pass be used to travel from Heathrow, Stansted or
Gatwick to London?

Yes, on National Rail trains, but as I think has been mentioned already, you can probably get a better deal by just buying a ticket from Stansted to Liverpool Street Station. Your onward trip from that station to Paddington will not be by train (you'll take the Tube, a Tfl bus, or a taxi) so a BritRail pass won't help with that bit anyway.

Does the Senior discount only apply to first class?

If you mean for the BritRail pass: no. When you go to one of the seller pages from the main Britrail site, enter your age as a "non adult" with your age (ACP rail) or select "senior" (at least one other - I didn't explore them all), and you'll see options for first and second class passes.

For the Senior Railcard that you'll buy when you get to the UK, also, no. See this page for a list of the tickets that are eligible.

Can you just show your pass or do you have to exchange it for a ticket
on each leg of your journey?

I used a BritRail pass extensively a few years ago. Mine was a FlexiPass (not consecutive days). I had it validated and then showed it to station staff as needed to get in or out through the barriers, and to the person checking tickets on the train. I also showed it to station staff at a ticket window when I reserved seats (I had a first class pass) for upcoming long journeys. (I think they issued me a paper seat reservation card, it may have been printed on the ticket stock, but it wasn't a ticket for the trip, just my seat reservation info.)

As has been said, with the help of the National Rail Journey Planner at www.nationalrail.co.uk, you can decide what's best. The value of the BritRail pass for me was the flexibility, especially for long train trips. I changed my itinerary at the last minute without financial penalty. For shorter trips, where your itinerary is fixed in advance, you may be better off using a Senior Railcard and advance purchase tickets. As I understand it, you can book and pay for the advance tickets at the Senior Railcard rate from home, and then buy your Senior Railcard before you pick up and use your tickets once you land in the UK. Be sure to search on nationalrail.co.uk using the Senior Railcard option, it's on a dropdown menu on the initial search page. ( I recommend you avoid the website nationalrail.COM, which is a reseller. Nationalrail.CO.UK inks to the train operating companies so you buy direct with no reseller fees.)

I wondered if I could purchase the new digital Senior Railcard before leaving home. I emailed the company and got this prompt reply:

Thank you for your email about purchasing a Digital Railcard. This is
available overseas however this can only be done over the phone as the
website doesn't allow you to change the country of the billing
address. We would need you to call us and create an account and then
send us in a photograph. Once that has been uploaded we will get in
contact with you to complete the Railcard application. Our Customer
Support Team number is 03453 000 250 (open from 7am – 10pm, seven days
a week). Alternatively, you can purchase a one year Railcard at any
manned station once you arrive into the UK.

I decided not to bother with the digital version. However, this information could be useful to someone who has a need to "hit the ground running" with a digital Senior Railcard already set up before arriving in the UK. That would not be my situation, though.

Can a Brit Rail pass be used to travel from Heathrow, Stansted or Gatwick to London?
Yes, on your counted BritRail Pass travel days (either the flexi pass or the consecutive version). Since you're not traveling in Scotland or Wales, you'd only need the BritRail England version.

Does the Senior discount only apply to first class?
BritRail Passes offer Senior discounts for ages 60+ when you buy either a 1st-class or a 2nd-class pass. We're usually happy with 2nd class. That's built into the price of the pass, based on age alone; it's not related to purchasing a Senior Card that gives you discounts on regular, point-to-point train tickets. For instance, BritRail England for a Senior in 2nd class currently costs $224 for 4 days within a month or $253 for 8 days consecutively.

Can you just show your pass or do you have to exchange it for a ticket on each leg of your journey?
You just show your pass onboard the train, and sometimes at a turnstile before entering the train platform. No separate tickets or seat reservations are required, although optional seat assignments can be requested for free at a train station up to a day ahead, for instance when you get the pass activated the first time. This makes the pass more flexible if you don't want to be committed to particular departure times.