The OU MBA is double (or triple, I can't remember) accredited. If an MBA is AMBA accredited, it'll be internationally recognised.
As SEHughes said, go with the one that best suits your circumstances. There are all kinds of MBAs, full time (12 months) part time ones where you just attend at weekends etc etc and the fees range from about £14,000 up to around £50,000 so there are plenty to choose from!