Sorry, No. Brighton is further from Victoria than from London Bridge.
In summary, Brighton is 50 miles 48 chains (50.60 miles) from London Bridge (buffers to buffers). Windmill Bridge Jn (just North of East Croydon) is 9m 65c from London Bridge, but 10m 00c from Victoria, so an extra 15 chains from Victoria.
But in detail, it's much more complicated than that, and has involved much research by members of the Railway Performance Society over the years! For starters, there isn't a single milepost series the whole way from London Bridge to Brighton, the SER series from Charing Cross taking over between Stoats Nest Jn and Earlswood on the original route via Redhill, before reverting to the London Bridge series. The Quarry line, used by most fast services, is 6 chains longer than via Redhill.
The London Bridge route splits to use two separate viaducts in the Bermonsey area, with the down 'slow' line peeling off to diveunder the new Thameslink viaduct and potentially a tad longer. There is a 5 chain shortfall at Bricklayers Arms, with milepost discrepancies thereafter. The buffers at Victoria platforms 16-19, used by most fast services, are at about 0m 08c.
The current RPS distance chart shows that a 4 car unit leaving P16-19 at Victoria, using the fast lines and the Quarry line, will travel 50.77 miles (decimal) to Brighton. Deduct 4 chains (0.05 miles) for 8 car and 8c (0.10 miles) for 12 car. For London Bridge to Brighton I recon 50.50 miles (decimal) for a 12 car Thameslink service from platforms 4/5 at London Bridge, slightly less from the bay platforms.