1st time with Virgin

Hi all
1st time flying with Virgin and 1st time posting on this forum so still trying to find my way around
We (hubby & I) are flying to Cancun in September and have upgraded to PE.
Just wondered what seats are best to book and whether to book upstairs or down? (747 LGW config 1)
Also - ?| could be a stupid question.... what is the relevance of SEQ - seen a lot of reference to this in reviews and online checkin but not 100% sure what this means.
Thanks in advance
1st time flying with Virgin and 1st time posting on this forum so still trying to find my way around

We (hubby & I) are flying to Cancun in September and have upgraded to PE.
Just wondered what seats are best to book and whether to book upstairs or down? (747 LGW config 1)
Also - ?| could be a stupid question.... what is the relevance of SEQ - seen a lot of reference to this in reviews and online checkin but not 100% sure what this means.
Thanks in advance