Bloody hell, David... That's a bit crazy! I've just had a quick check on-line for taxis, and apparently a taxi from Luton to Oxford will cost about ?85 (and you'd have to get back, too!).
Maybe some fan travelling from the Luton area would be kind enough to give you a lift?
Alternatively, you can hire a car from Avis (
www.avis.co.uk) for a lot less than a taxi: their site quotes ?35 for a day (for a Corsa 1.2 LS). Though if you did that you'd have to find somewhere to park in Oxford (and pay for it)!
It might even be easier to let the flights go and try to book another flight into Heathrow; there's lots of buses and trains from there to Oxford, and they're pretty cheap.
Hope you can sort it out: my wife and I will be there, and it'd be great to know that we'll be meeting someone who isn't a stranger!
Cheers,
Mike C