More relevant is whether obtaining any OpUp or other special treatment may be a criminal offence. The most obvious offence would be fraud by false representation (s2 Fraud Act 2006). It's possible there may also be an offence of obtaining property by deception (s15 Theft Act 1968) (but it's complicated by whether an OpUp is considered 'property' and whether permanent deprivation can be for the duration of a specific flight), but it appears that the offence of obtaining pecuniary advantage by deception (s16 Theft Act 1968) does not apply (because the scope of is effectively limited to (i) banking / insurance-related issues; and (ii) employment).