Update: you can take the supergirl out and keep green arrow as well and then take the rest to CEX for £27 cash. They need the Starter Pack to be sealed, but the most outside carton box can be opened, as they will open it anyway to check the inside parts when you trade in, as long as the game case and the inside lego carton box are still sealed (maybe you also need to remove the supergirl sticker on the outside box carefully without damaging the box too much in case it would confuse CEX and make them think supergirl is also an essential part of the Starter Pack. This is not about cheating CEX as they will probably pay you the same no matter whether it is a supergirl version). It means that by this way you can get supergirl+green arrow for £38, and of course supergirl alone for £2.99 if you manage to get the supergirl starter pack from Smyths.
The official Green Arrow and Supergirl minifigures have golden (or maybe you can call this color orange) bases instead of the cheap blue ones.
First read an article about the Lego Dimensions Starter Pack Green Arrow promotion at GAME from @BricksFans on Twitter, and then went to the local GAME store at Angel Row, Nottingham to try my luck. Although they didn't put Supergirl Starter Pack on the shelf or have any poster about the promotion, the gentleman at the counter told me they had the limited edition Green Arrow Lego Dimensions minifigure and they also helped me to find the Supergirl Starter Pack in their storeroom. I know £65 is a much higher price compared with the deal offered by Smyths Toys or even the Sainsbury's, but if you want to complete your lego dimensions collection and you didn't have a chance to go to US and don't trust those on ebay, maybe it's worth a try.
BTW, you'd better ask their staff at counter directly instead of a random guy working in the shop since the latter may not know about the promotion.
As far as I know, supergirl and green arrow are to be released publicly someday (or have been released already maybe. I haven't really started to play the game..) but as in-game characters which you can hire for 30s, with no solid information about public release of these physical minifigures apart from the GAME promotion and the give-out at Brick Live in Birmingham late this October.