Keeping this as simple as possible...
HMV's Re-Play App for iPhone scans Bulletstorm Epic Edition (it has to be the Epic Edition, not just a standard one) at £30 credit, £25 cash.
When you go in store they'll scan it at £12 credit, £8 cash. Show them the App price and they'll grumble (may even call head office) but they should honour the trade in/cash value.
I just got Duke Nukem Forever for £9.99 as a result.
Good way to get rid of Bulletstorm (Epid Edition) for a seriously good price (considering how old it is) and get a new title or some cash...
Heard staff say Head Office wont be able to resolve til next week so get in quick!
Hope it helps someone/anyone!