Hello, I'm new here and hope I have the correct thread. I'm travelling from Italy to London Heathrow T5 on a weekday arriving late afternoon. I have already bought a 1st Class BritRail pass to travel on from Kings Cross to Grantham. I think I will use the Underground (I have a prepaid Oyster Card) as it's direct to Kings Cross and I think I will allow 2 & half hours from when I leave Heathrow to reach Kings Cross. My question is I know I can pay an extra 4 euros to reserve a seat, but I'm in two minds to do so. I suppose I will arrive around 7pm, and thinking it might be peak time might be very busy, does anyone recommend reserving seats in 1st class or should I take the option of using the 1st available train departing from Kings Cross without time constraints or second guessing how long it will take to get out of passport control etc. Thank you very much for any suggestions!