Spent the day on the end of platform 1 on Monday 11th October. No problems of access, ask politely, state that you are trainspotting and photographing, smile and you are on.
From plat 1 you need bins to see the local unit numbers and coaches across the station. Plenty of action with 91's, HST's, 317,s, 365's and the odd 313's. Surprised at lunchtime by the Yellow Peril HST Track recorder. Thunderbird 67008 parked on siding near the tunnel.

Kings Cross is a fantastic station for spotting, but, I found it very dim and poorly lit so photography was a bit of a problem. Best to go to the end of the platforms to take shots there. I found most of the staff very unhelpful and gruff the day I visited, being told by one member of staff that if I took photographs then Network Rail Security staff were likely to remove me from the station! (I found that rather odd, as on their website I found a piece to say that they welcome rail enthusiasts in all their stations!)
Just find an un-manned barrier and make a dash for it to the end of the platforms (8 and 9 are best because you can access suburban services from there!) and you should be O.K.
Food and drink is plentyful here (take plenty of cash) and all the usual facilities are in the concourse.
Best to travel here by public transport as parking is virtually non-existent.
Plenty of (GNER) HST 43's & 91's and Hull Trains 222's to see on platforms 1 to 8, where 9 to 11 service suburban services with loads of 313's 317's 365's etc.
Started here as a kid, so sentimental value puts it in the ratings for me at least, of 8½/10.

Instead of going to Kings X on a weekday, go just up the line to Finsbury Park. You will see all the Kings X trains plus the frequent Moorgate (Hornsey)313's

Plenty of catering outlets outside and in,some on platform. Good views from end of the platforms. Best to travel there by public transport

Public transport is London buses and the London underground
(Northern Line)

Great place to see and photograph HST's and class 91's
PLUS an EWS 47 and/or class 67 are stabled in the sidings at the end of the platform (to your right as you enter the station)