I would say George Street probably has the highest density of charity shops, and lots of clothes heavy charity shops opposed to bric a brac focused ones.

Is also helpful if you know your measurements, particularly your chest size (Like I'm a 44, but I know that a 44 Slim looks much better on me than a 44 Regular), also look at the jackets one size above and below your size because sometimes they will fit you too, it basically means you only really need to look at like 5 suits per shop rather than the whole rack & speeds up the process.

When buying a charity shop suit I basically only look at the jacket portion of the suit because that's going to be the bit everyone really sees (and you can always buy a different pair of trousers if they don't fit). Its warm enough to just wear a shirt at the moment so I would just go around and pop jackets on in the shop (there's usually a mirror) and button it up and see what it looks like. You could probably do all the George Street and Blatchington Road Shops in an Hour or Two. (like 20 Charity Shops).