Day trip is really pushing it, given time of match. Don't know if you can get from Elgin to Airport after 5.00pm in time for last flight. Not saying it's impossible but you'll need to check. Find out time of last flight to London and I'll check buses/trains. Unfortunately there is no overnight sleeper train to London on a Saturday night. There might also be late changes to match time - because The Rangers are involved and TV may be interested. I would seriously consider overnighter. Good luck.