M1 then across to the M40 - brings you straight to Wembley or many stations on the Chiltern Line with direct quick access and easy parking - no need to drive inLondon.
If people do come down the M1, I'd definitely recommend this, as it avoids the nightmare around Northampton, Luton etc. You can either take the M69 and then the A46 past Coventry, or leave the M1 earlier and take a slightly bigger detour down the M42.
All depends how the M1 is likely to be between 23A and 21. If it looks OK, I'd stay on until 21 and go M69.
M6, M42, M40, High Wycombe, Train.