If our trains run to time we should arrive at Heathrow 3.5 hours before the flight. Plenty, surely, even allowing for some train delays?
Yes - this is plenty of time
We've packed soft rucsacs that are well within the weigh allowance, but if squashed the wrong way might be slightly outside the dimensions. I mean an inch or so. What happens if they're not OK?
Unless they’re full to bursting, you’ll be ok
I have booked seats (free) but not food for the flight. I'm hoping we'll have plenty of time to eat at the airport, to kill some of the time? If all else fails can we buy sandwiches at the airport and take them on the plane?
yes you can take food on the plane with you - just pick up a meal deal from Boots.
Similarly, the fight is mid afternoon, but we won't get our evening meal until late evening. Am I allowed to take nuts or chocolate bars to tide us over? Not EU country, regs seem to be no fresh food.
Buy something at the airport when you land?
If I buy e.g. food or duty free, a book at the airport, does that need to fit into our regulation sized bags or is it extra?
No. It’s extra. But if there’s delays getting through security, you might not have time to buy anything.
We need to take coats. Does it matter if they don't fit in the bag? I.e. we'll be "wearing" them but unlikely, looking at the weather forecast, to have them on at departure. We will need them at the other end.
Just pop it in before you go into the plane.
I know we'll have to empty water bottles before security, is there somewhere to refill before getting on the plane?
Yes. Airports tend to have water fountains airside these days.