SAVE CONGESTION ZONE FEES! There is an NCP underground car park at 170 Marylebone Road, NW1 5AR (close to the intersection with Gloucester Place and midway between Marylebone and Baker Street tube stations). For details, click here. This car park is NOT within the congestion zone and it is only a brief walk to St Mary's church from there. There are 150 parking spaces in this car park. You may also be able to find pay-to-park parking spaces in the streets surrounding the church but using these WILL incur the Congestion Zone charge.
There is also a National Car Park at 43A Crawford St, London W1H 1JR and this one is definitely inside the Congestion Zone.
The Central London Congestion Charging Zone requires that drivers travelling into Central London pay a daily congestion charge, so be careful where you choose to park. You can download a map of the Zone here. Currently, zone charges are not made on Bank Holiday weekdays (eg: Monday 30 May), but will definitely be applicable on the wedding day. Currently, the charge is around £10 when paid the same day, or paid in advance, and £12 if paid by midnight on the the first charging day after travel - which will be Tuesday 31 May 2011. If you are registered for Congestion Charging Auto Pay, the charge will be £9.00. For up-to-date information about congestion charges, please visit this link.
As an alternative, you might want to consider leaving your car at your hotel/accommodation if it is outside the zone, or even at the reception venue (The Grange does have some free parking) and then travelling into Central London using public transport.