I am travelling with a group of 12 in late June/early July. All travelers have Britrail passes. I am wondering how necessary it will be for me to arrange seat reservations for each traveler ahead of time. I have done so for one of our journeys, from London to Hull, but I would like to leave the itinerary for a few days open. It seems like trains run so frequently that it won't be a problem, but would like to know more about what to expect.
Seat reservations are not compulsory, so if all seats are full you can still get on but you'll have to stand.
It all depends when and where you're travelling. Long distance you should consider reserving at busy times if you want to guarantee to sit down.
If you don't reserve, you can expect your party to have to find vacant seats throughout the train. If you reserve for a party, they should bunch all your seats together. As has been said, it all depends on where & when you are travelling.
Getting on trains will not usually be a problem, but getting a seat could be, depending on the day and time of travel. Generally, if you are likely to be a commuter train, i.e. weekday between 6.30 and 9.30 or 16.00 to 19.00, getting a seat could be a problem without booking and you will certainly not get 12 seats together.
I have had to stand before now from York to London and from London to Manchester on a Friday night. Both are 2 hours+. If you are getting on a train at the start of its journey, it will be easier to get a seat.
Tickets are usually cheaper if you name the train you will be on, but these do not give any flexibility and if you miss your named train, you have to pay again. Travelling at the weekend should mean that you can all get seats. Check for engineering work if using the service at a weekend as there maybe bus replacement services which will delay your journey.
Sometimes there are several operators running the same route, but some will be express trains that are quicker and will cost more then the local operators. The fast trains are usually busier.
An itinerary would be helpful to have a guess at whether a particular journey at a particular time would benefit from reservations.
Some routes may have high frequencies, yet be formed of only 2-3 carriages.
With your BritRail Pass, you can get free seat reservations up to a day in advance at British train stations, if your plan is more settled at that time. If several people are traveling on one group pass, they should attempt to stay together, or at least to know where each other are, until the conductor has checked tickets and passes.