This is a recipe for trouble imo.
You need to be calling for start really no later than 17:40 to have a sensible chance of starting up, warming up the engine, setting up navaids etc, taxi, power-checks, getting your departure clearance, lining up and being airborne by 18:00.
Before this you need to get through booking out, security if they have it at Exeter, paying any fees such as parking etc, pre-flighting your aircraft (what if a clumsy muppet walks into your aircraft while you're at the match and damages the tail plane or perhaps a nav light? You can't skimp on pre-flighting) and loading your passengers. I would allow 30 minutes minimum for this lot, so now we're at a required 17:10 arrival at the airport.
You're assuming the taxi will be on time dead on 17:00, that you and your pax will be able to get to the taxi by dead on 17:00, and that the traffic just outside the stadium, just after a game, will be predictable. It could be 15 minutes or it could be an hour, particularly if there are road closures associated with the game.
If you did get to the airport say by 17:30, would you be tempted to skimp on something to make the 18:00 deadline? Maybe don't do full pre-flight? Maybe don't do full power checks? Get-there-itis is not clever, but always makes for an interesting read in AAIB reports, if a little sobering.
Personally I wouldn't recommend another away match cos I dislike football, but you could try rugby instead.