My understanding is that Off-Peak UK National Rail train tickets can be used on any train serving the selected route, as long as the travelling time window fits in the off-peak definition. Or
Buy anytime, travel off peak, as National Rail puts it. Similarly, Anytime tickets allow passengers to travel at any time, as the name suggests.
However when I was booking my Off-Peak ticket I was asked if I wished to reserve a seat. Wouldn't that go against the whole principle of travelling any time provided you travel off-peak? In other words, does reserving a seat linked to an Off-Peak/Anytime journey force me to travel on that particular train, or am I still free to pick the one that suits me best?
Just to add some context, I'm looking to book a fairly complex train journey with 3+ connections, crossing 2 countries. I am therefore worried of delays which might cause me to miss my connecting trains. My rationale would be to buy Off-Peak tickets so that I can travel on any train, thus removing delayed connections from the equation. Upon booking I saw that both Advance as well as Off-Peak tickets can be purchased. I'm worried that reserving a seat would invalidate my right to pick any train I wish.