There are a number of semi-fast trains that start from WGC and call at Hatfield, Potters Bar, New Barnet, Oakleigh Park, possibly Alexandra Palace or New Southgate and then Finsbury Park and King's Cross.
These should be 6 or 8-car trains and if the latter, most likely first class is declassified (but don't tell everyone!). In other words, find the right services and you'll be just fine I think.
I sometimes get the 0818 or 0846 services from Hatfield (not sure what time they call at Oakleigh Park) and I can assure you that you could get a seat, or if you don't fancy that (some rolling stock is 3+2 seating and can get a little cramped), then there's loads of room to stand without being squashed up. Go earlier, and I'd imagine the trains are even emptier, given that you'll be arriving at King's Cross around 0820-0830 max, which is quite early for most people getting to work - even with an ongoing tube journey.
Failing that, you have the slow services to Moorgate that should be okay too.
To be honest, on the inner-suburban stations you're bound to be okay at pretty much anytime. It's the people coming from Cambridge and Peterborough that seem to suffer the most from overcrowding.
Obviously, if services are cancelled, short formed or they have to run an emergency timetable, then it's every man for himself! At least you have the option of getting a bus to Oakwood or High Barnet or Totteridge & Whetstone if it all goes pear shaped.