Egham is a safe area, your car is insured, and don't leave any valuables inside. Really can't see an issue. I park very often on the street to use public transport free or use airport parking. Obviously don't block someone's drive, but if you are in a low profile car then really what's the problem. If you do it regularly, avoid parking at the exact same spot as well, a bit of tactical evasion is all that is needed. I've had my car broken into parking for 20 mins in Knightsbridge, but left untouched all day and night long when parked next to a train station where it was obvious I was parking for free to not use the station parking!

I'd easily take the 'risk', and 150 GBP is not exactly nickel and diming!