Upvote:10
If you want to have a good chance of getting four seats together, never mind a table seat, you'd be well advised to reserve where possible. Reservations do not commit you, so if you missed the train with your reserved seats, then no big deal (although your reservation will not carry over). If you are travelling in first class, then on most long distance journeyes, all seats have tables.
Note that despite you reserving seats, people may sit in the seats you have reserved anyway - this is because as mentioned above, reservations are rarely commitments. If rail passengers avoid reserved seats, then you'd have trouble on busier trains. If this has happened, simply politley ask the people in your seats to move, and they almost certainly will.
As for reserving specific seats, I'm not sure how it works with Britrail. I do know that if you buy your tickets directly from Crosscountry or Virign trains, and are travelling on a service of that operator, the website sometimes show you a train layout, much like airline seat selection. If you're reserving by phone, you can always tell the operator your preferences.
I should mention I have little direct experience of 1st class - I've had a ticket for it some 3 or 4 times. Behaviour and etiquette may be a little different to the (usually comfortable on long distance routes) standard class.