Consent from the superior landlord to let is not unusual, but I suggest you get legal advice. If the consent to let is included then I suggest (1) that there should be a provision that consent should not be unreasonably withheld and (2) that the let should be for a minimum of say 3 months to prevent Airbnbs. I know that may cut both ways if you want to benefit from Airbnb, but on the other hand just think how your quality of life might be impacted if the other flat was filled with a revolving population of strangers.
I you do want legal advice then try Gregsons in Wimbledon. I have used them for over 20 years, including on property matters, and have always found them sensible and efficient.
Hope that helps.