It sounds like you have the convenience of being able to easily park your car at work. I'd have no chance of getting a parking space at work, and would probably have to pay in excess of £20 a day to park in a car park (I work in South Kensington).
With approximately 240 working days a year, and assuming an unreasonably low parking cost (say £15 a day), that's £3600 a year. You can pretty much ignore all the other costs in comparison.
If I travelled by public transport, I could mostly get away with a Zone 1-3 Travelcard (Tramlink to Croydon using Oyster works if your Travelcard covers any of Zones 3 to 6, even though the Croydon stations are in Zone 5), which is a mere £1368 annually.
For any of those costs, I could buy a new bike every year, throw it away at the end of the year, and still save money (assuming I didn't buy a carbon fibre racing bike, anyway, and kept the price reasonable).
All that aside, my daily commute is 23.2 miles, so for the 240 days I quoted above, my annual mileage is 5568 miles, so I do 7000 miles in just over one year, 3 months and 2 days. It wouldn't take me long to pay off a new bike based on your costs (and this assumes I don't use it all for any other purposes, like shopping, picking up parcels etc).
If the Congestion Charge still covered this area (which it did for almost four years), then that would be another £8 a day (add on £1920 to the cost).
The other flip side, is that if I come in on the Tram/Tube I read more, so there's an additional cost of books on my Kindle.