Yep. Opposite the mailbox is a good place to start - you can get good light trails in with the glow of the main building. You'll need a bit of trial and error on the exposure. From the Mailbox side you can get good shots of the new Radisson Hotel.
Otherwise, try down the back end of the bull ring by the markets to photograph the Selfirdges building - you may get moved on by over zealous security guards though. Down by Moor Street station is another good vantage point or by the entrance to the car park.
Finally, Colmore Row and St Phillips Sq are great locations - plenty of places to set up to take photos of the cathedral.
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Just to add, minus the scaffolding and with no market to mess up the scene, check out Victoria Square (top end of New Street). No experience of high rise but there is an NCP opposite the Mailbox that would be an excellent vantage point.
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