Never been to NCL so I can’t comment, but spotting facilities at MAN and BHX are very good. Both have an interesting mix of aircraft (MAN being busier). LHR and LGW have even more diverse types and there are good vantage points around the airport, but there are no dedicated facilities for spotters within the airports and the attitude of the BAA to spotters isn’t exactly positive 🙁 . LTN is good for bizjets and there are good vantage points within walking distance of the terminal. Likewise, LCY is good for props and smaller types and the best vantage point is adjacent to the terminal.