There is a bus service from St Albans Abbey that goes near the ground and main station. Also Finsbury Park route to Hatfield which also has a direct bus to St Albans.
Bus replacement is 12 miles from Mill Hill to St Albans, on a Saturday that doesn't sound good!
I've looked into the Mill Hill bus replacement service, it is every 30 minutes, with a 52 minute journey from Mill Hill to St Albans. The St Pancras to Luton line is busy, hopefully the replacement bus service wont be so packed one cant get a seat.
To avoid that risk/ gamble I'm looking forward to the Radio Wings commentary!