I've used Britrail passes. Twice. The first time, it wasn't a great thing from the financial point of view - I could have taken the rail trips for less money. But I took the hit to my wallet without a flinch because it was convenient and stress-free just to go to the station and get on a train. The second time, it was one of my best bargain buys. Ever.
Not to mention, I'm a person of at least ordinary intelligence, with a decent amount of travel experience - and I find the British train ticketing system just freakin' intimidating. I swear, I spent a lot of time on internet forums, asking questions, and carefully reading the answers and other information. And yet all the talk about "advance tickets" and "anytime tickets" and walk-up fares and "you have to change at Little Fartwold, be sure to cross to the other platform" - made as much sense to me as "yada yada blah blah." You know, the cartoon where the human is speaking in complete sentences to a dog and all the dog hears is "blah blah blah"? Yeah. Like that.
So back in 2009 or 2010, I forget which trip, I got the Britrail London Plus pass and used it for day trips. I could afford it, and I really needed to relax on that trip. As travel costs go, it was a splurge, but not ruinous. And, it was fabulous to just get to the station and get on a train, and ride. Totally worth the cost.
Then, I found https://seat61.com/index.html - The Man in Seat Sixty-One. Bless that man's heart. His website cracked the subject open for me, and bits of comprehension began to filter into my brain. I kept reading the Q&As in travel forums.
My second Britrail pass was for a 2-week trip around the UK in late 2015. I covered a lot of ground, with long trips, and my itinerary changed a bit even after I'd landed in the UK. I visited Glasgow, Edinburgh, York, day trips around York, Penzance, London, Brighton. That pass saved me hundreds of pounds - which my itinerary changes would have cost if I'd bought those advance fare tickets instead of having the Britrail pass.
Now, with www.nationalrail.co.uk and https://seat61.com/index.html, I feel much more comfortable about going to the UK without a Britrail pass. But I did price it out for one possible future itinerary, and I wouldn't rule one out. By the way, I recommend that you avoid the site www.nationalrail.**com** which is the Trainline, a reseller site that adds fees to the cost of train tickets. As the Man In Seat Sixty-One says, you should buy direct from the train operators' sites. Details here: https://seat61.com/UK-train-travel.htm
So, C.M., if you're just doing day trips around London, to places with frequent trains, you'd probably come out a little better financially without the Britrail pass. But the convenience and flexibility is worth something, and after all you're on a vacation.
As to seat reservations, I know that with a First Class pass you can make reservations at least a day in advance of the trip, at a rail station. Not sure about standard class. You have to present your Britrail pass at a train station to be validated. If you have a list of your trips in hand you can get seat reservations at that time. For long journeys or peak times a seat reservation is in my opinion important. But they will load the trains up with everyone who has a ticket or pass, even if some passengers are standing. You will get to your destination but you might not get a seat. OTOH I didn't have reservations when I went to Bath and got a seat just fine.