ACP Rail

I'm not familiar with that website so can't comment. The main UK site is here to compare, check prices etc National Rail

You may like to consider Group Save but beware of the not before 9.30am rule - which does not apply if each person buys a Senior Railcard.https://www.nationalrail.co.uk/tickets-railcards-and-offers/saving-money/group-travel/

Also note that on some journeys, it is cheaper to split the ticket & the national rail website does not tell you this. This site does come up with 1 split:>https://www.buytickets.scotrail.co.uk/

These sites also come up with splits (if cheaper) but charge commission on amount saved:> https://trainsplit.com
https://www.traintickets.com/?/

Scotrail does not charge commission on Split tickets so they are the site which should always be used for such tickets.

The other two companies are external companies to the rail industry (ie- not rail companies). Why on earth pay commission to them when Scotrail do it for free?

To actually answer your question ACP are the US vendor for Britrail passes. Whether or not a pass makes sense ultimately depends on whether you want to retain flexibility (just hop a train when you like) or if you are willing to commit to the far cheaper Advance train specific fares ahead of time.
If you do the latter then Britrail is massively less likely to be a good option. If you are relying on the RS fare charts for your math, they take no account of Advance Fares- often now on many routes they are available (despite their name) until minutes before travel.

Trainline are another Britrail vendor-probably for a commission.

Britrail gives a 15% senior discount, any UK railcard gives 33%.

If you are a group, rather than buying a senior railcard each for £30 (soon to be £35) you could look at buying Two Together Railcards for £30/£35 for the two of you (another saving).
On Two Together you can't travel before 0930 on a weekday (any time weekends) but then you can't travel before 0930 in London and the South East on a senior railcard either on a weekday (to protect commuter revenue)

We used ACP Britrail Pass on our last trip to the UK (2023) and it worked very well for us. We did the calculations in advance also and they were accurate. We saved quite a bit on train tickets. I plan on using them again.

I always use them. Go to the BritRail website and pick a pass. When you go to buy it, you have to choose a vendor. I use "ACP Rail for Americans". Never a hassle, and the passes arrive pronto.

My wife and I go to England a lot, and we take the train everywhere. We always get a rail pass even if it costs a little more than the separate tickets would. The convenience is worth it. You can just hop on whatever train you want whenever you want.