London on Sunday

Hi, We are a family of 6, 2 Adults, a 17 year old and kids aged 12, 7 & 4. We are planning on coming into London from the West (Basingstoke) on Sunday to do the 'Sights'. Have decided driving in is cheaper than Train. Can someone suggest a free parking area. Also any comments on 'The original London sightseeing Bus tour' as we are considering this.
Many Thanks

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