You may have some anecdotal evidence that extra parking in Brixton benefits the market, but it is unlikely to be the case in my opinion.
Traders (the large majority, everywhere as far as I can see) routinely fight attempts to cut down on car use, claiming that it will reduce their trade and cost them money. However whenever the claim has been systematically analysed it has found to be false. When Oxford pedestriansed some city centre roads in 1999, te storm of opposition was so strong that the council agreed to monitor trading levels over the next two years. The report was carried out by C.B.Hillier Parker, the property and retail consultants and agents. They found that far from deterring shoppers, footfall rose in pedestrianised streets (by 10%) and that commercial rents in pedestranised streets had begun to grow quicker than in non-pedestrianised ones. The same result was found by a Living Streets analysis of pedestrianisation in York.
Why do traders get it wrong so often? One reason is that they are often highly car-dependent themselves (especially small independent businesses that organise their own stocking). Another is that they routinely make false assumptions about their customers. Sustrans have carried out a study matching traders beliefs with customers actual behaviour in two areas of Bristol (
http://www.sustrans.org.uk/assets/files/liveable neighbourhoods/Shoppers info sheet - LN02.pdf). It found that traders believed that just 12% of their shoppers lived within half a mile - in fact 42% did. Traders believed that 40% of their shoppers came from more than 2 miles away - in fact 86% lived within 2 miles. They also completely overestimated car use amongst their shoppers (believing it to be 41%, actually 22%) and wildly underestimated all other travel modes, especially buses and foot.
One particularly interesting result was that car drivers and cyclists made 4 times as many single-stop visits as pedestrians and public transport users - ie just went to one shop - suggesting that many are "drive-thru" shoppers stopping on their way elsewhere. Pedestrians and public transport users were far "better" shoppers, using several different shops on each visit and spending proportionately more. In fact, given that we know that cars drive down consumer footfall, the extra spend that they generate may well be smaller than the trade they drive away.
We don't have any hard data on Brixton, which is a shame and it should
definitely be collected before - for example - any decisions are made about expanding Tescos and its carpark, or spending millions shoring up Popes Road multistory. The money would almost certainly be better spent elsewhere (assuming we care about the helath of the market and its traders).
Tesco's in particular have been quite active in pushing the myth that extra carparking will aid the market; the opposite will almost certainly be true.