On Sunday 3 December Avanti West Coast have an ASLEF strike- no trains are expected to run at all.
By unfortunate coincidence LNER have engineering work on that day which means that no trains can run into Kings Cross. There will be a bus transfer to Cockfosters for London Underground trains. The KC closure applies to all operators including Lumo and Grand Central, also trains to Cambridge).
On Saturday 2 December the LNER strike means that the limited number of trains running will be very busy.
So best to travel on the Friday or Monday (both of which days will also be busier than normal) or use other modes of travel.